Using the legal definition of rape, the "National Intimate Partner and Sexual Violence Survey (2010)", a large (more than 18,000 people interviewed) survey by the CDC, comes to the conclusion that the risk to fall victim to a rape / an attempted rape / a drug induced rape in 2009 was about equal for men and for women (both 1.1%). For men the chance that the perpetrator in that case was a woman was about 80%. A ballpark number for the sex of the perpetrators tells me it is a 60/40 split, meaning about 40% of rapes / attempted rapes / drug induced rapes have been committed by women.
Clarifications, citations and calculations:
As this was not a prominent finding of the survey (as a focus on male victims / female perpetrators is seldom in the spotlight) and the CDC failed to define female-on-male rape as rape, I have to explain in detail where this data is coming from.
Consider the following example:
*** Trigger warning for rape scenario ***
Scenario A: Jane passes out drunk at a party. John has sex with her.
Scenario B: Jack passes out drunk at a party. Jill has sex with him.
*** Trigger warning for rape scenario ***
Both scenarios are an equivalent to each other and to my best knowledge both scenarios are considered rape in most parts of the USA. Via the CDC definitions, Scenario A would have been considered as rape, while Scenario B would have been considered "Other Sexual Violence - Made to penetrate". Legally speaking, I am comparing oranges with oranges here, even though the CDC pulls the "that wasn't rape-rape" card. Now if you compare the percentage of rape for women with the percentage of "Other Sexual Violence - Made to penetrate" (=rape) for men (again, both numbers include attempts as well as drug induced rape) the number in the last year for both is 1.1%.
As for the other numbers, 98.2% of women have only been raped by men and 79.2% of men have only been raped by women. Via these numbers, one can calculate the split of the perpetrators (my ballpark number).
Most of the discussion around these numbers involve the difference between the lifetime numbers and the numbers for the last year. Looking at the lifetime numbers we get a 18.3/4.8 (female/male) difference. What is going on here? The lifetime numbers are a reflection of the past, while the numbers of last year show us what is going on recently. None of us can affect the past, but we all can affect the now and the future with our actions. Only looking at the past (lifetime) figures while ignoring more recent figures is a poor baseline for making good choices now. Several explanations can be made why the numbers differ so much. For once, one can argue that women have become more aggressive (there are more female-on-male rapes now) or/and that men have become less aggressive (there have been more male-on-female rapes in the past than now) or/and that the male lifetime numbers suffer from a poor understanding of male victimization in the past (the male lifetime number is underreported as the view of male victimization is changing). People tend to recall events taking place within recent history better then events that have taken place more distantly in their memory. Thus statistics regarding recent history tend to be more accurate.
At least one other study has found comparable victimization rates.
 - Wikipedia: Laws regarding rape - There is no national rape law in the United States, due to the United States v. Morrison ruling that parts of the Violence Against Women Act of 1994 were unconstitutional. Each state has its own laws concerning sexual aggression. Nor is there any national standard in the US for defining and reporting male-male or female-perpetrated rapes. In most states, the definition of rape is broad, with respect to genders and the nature of the acts involved. Info on the 4 largest states (containing 1/3 of US pop): (In case this reference is questioned/challenged as inaccurate.) None are gender-specific, all include CA: "A Penal Code 261 pc "rape" occurs...under California rape law...when an individual engages in sexual intercourse with another person when the sexual act is accomplished (1) against that person's will, or (2) without that person’s consent.." - http://en.wikipedia.org/wiki/Laws_regarding_rape
 - National Intimate Partner and Sexual Violence Survey (2010) - http://www.cdc.gov/ViolencePrevention/pdf/NISVS_Report2010-a.pdf
 - See table on page 19 and the definitions on page 17:
Rape is defined as any completed or attempted unwanted vaginal (for women), oral, or anal penetration through the use of physical force (such as being pinned or held down, or by the use of violence) or threats to physically harm and includes times when the victim was drunk, high, drugged, or passed out and unable to consent. Rape is separated into three types, completed forced penetration, attempted forced penetration, and completed alcohol or drug facilitated penetration.
Being made to penetrate someone else includes times when the victim was made to, or there was an attempt to make them, sexually penetrate someone without the victim’s consent because the victim was physically forced (such as being pinned or held down, or by the use of violence) or threatened with physical harm, or when the victim was drunk, high, drugged, or passed out and unable to consent. - -Among women, this behavior reflects a female being made to orally penetrate another female’s vagina or anus. - -Among men, being made to penetrate someone else could have occurred in multiple ways: being made to vaginally penetrate a female using one’s own penis; orally penetrating a female’s vagina or anus; anally penetrating a male or female; or being made to receive oral sex from a male or female. It also includes female perpetrators attempting to force male victims to penetrate them, though it did not happen.
 - Table 2.1 on page 18 and table 2.2 on page 19
 - For female rape victims, 98.1% reported only male perpetrators [...] a majority of male victims reported only female perpetrators: being made to penetrate (79.2%) - Page 24
 - Calculation of the ballpark number:
98.1 + 20.8 = 118.9 male rapists
79.2 + 1.9 = 81.1 female rapists
81.1 / 200 = 0.4055 -> 40% female rapists
Limitation of that number. We can not be sure that the 1.9% and the 20.8% include ONLY male/female perpetrators as we do not know the figure of people who have been raped by both men and women. Also, even though most victims of both sexes report only one perpetrator we can not be sure how people who report multiple rapes would influence the number.
 Women are increasingly taking on a role where they are the sexual aggressor/sexual initiator. That role does not come without risks and one of them is presuming consent where there is none. The narrative that the overwhelming majority of rapist as well as the insistence (of media, law and surveys) of categorizing those actions by women as something else than rape only increases the risks that these women will at some stage rape a partner since they are less likley to evaluate their own behaviour because why make efforts to avoid something that almost never happens and which only truly evil girls do. See also From Deviance to Normalcy: Women as Sexual Aggressors (2002) - Studies have revealed that young girls have become socially assertive in calling young boys on the telephone and even asking for dates at a very early age (Anderson, Arceneaux, Carter, Miller, & King, 1995). Also, women are now expected to take an active role in sex (O'Sullivan & Byers, 1996), and are expressing themselves sexually in aggressive behavior patterns (Anderson & Struckman-Johnson, 1998). Rates of sexually aggressive behaviors among women vary from one segment of the United States to another, but the evidence presented here shows that as many as 7% of women self-report the use of physical force to obtain sex, 40% self-report sexual coercion, and over 50% self-report initiating sexual contact with a man while his judgment was impaired by drugs or alcohol (Anderson, 1998). Given these numbers, it is appropriate to conclude that women's sexual aggression now represents a usual or typical pattern (i.e., has become normal), within the limits of the data reviewed in this paper. - http://www.ejhs.org/volume5/deviancetonormal.htm
 - BJS - NCVS Rape Rates - http://www.bjs.gov/content/glance/rape.cfm
 - Men are becoming aware of this issue and to a lesser degree buy into the narrative that men should welcome all sex. This notion I suspect also explains some (I don’t have any idea of how much) of the difference in lifetime numbers as elder men are more likely to recall past events as being consensual when they in fact were not. In order to recall them as non-consensual one must recognize that one have the option to consent or not. / Researchers into the field of traumatic memory recovery note that the longer the period of time a person is asked recall a traumatic event, the less likely they are to remember it. How this works is that surveys that ask about a traumatic event in the last six months get less false negatives than those that ask about a traumatic event in the last twelve months which, itself, gets considerably fewer false negatives than lifetime prevalence. For men this effect is even more pronounced. "16% of men with documented cases of sexual abuse considered their early childhood experiences sexual abuse, compared with 64% of women with documented cases of sexual abuse. These gender differences may reflect inadequate measurement techniques or an unwillingness on the part of men to disclose this information (Widom and Morris 1997)." [...] Comparing the lifetime rate of sexual abuse for men and women is misleading in determining their relative risk of sexual violence, simply because men disclose childhood sexual abuse four times less often than women. There may be many reasons for this. It’s unlikely that it’s due to sexual abuse being less impactful on men because studies have shown that sexual abuse does have a profound impact on men, and this includes female-on-male sexual abuse. For instance, the link between sexual abuse and suicide attempts is stronger in boys (Rhodes et al. 2001) and sexually abused boys are twice as likely to commit suicide (Molnar et al. 2001) than sexually abused girls. In addition to that, there is a risk factor for sexually abused men to sexually abuse others is if their abuser was female (Salter et al. 2003). One possible reason for men not disclosing, or even “forgetting”, is quite simple: our social narrative does not allow for, nor does it depict, the sexual abuse of males. To a degree it allows for the sexual abuse of boys by men, but not boys by women or adult men by anyone. In a study on the effects of retention interval and gender on the perception of violence, Ahola et al. (2009) found that eyewitnesses rated female perpetrators less violent than male when reporting after an interval of one to three weeks as opposed to ten minutes. Ahola et al. (2009) proposed that over time eyewitnesses reinterpreted the behavior of perpetrators in order to conform to gender stereotypes regarding violence. Widom and Morris (1997) propose that a similar process is occurring with male victims of sexual abuse (particularly by females) as, over time, they reinterpret their victimization to conform with the dominant social narrative regarding sexual abuse: that it happens to women and is perpetrated by men. They will do this by reframing their abuse as consensual or as a rite of passage or less violent than it was or by “forgetting” it completely. The more time passes, the more our memories conform to the dominant social narrative. "Gender differences in reporting and in perceptions of early childhood experiences may reflect early socialization experiences in which men learn to view these behaviors as non-predatory and non-abusive. Many of the sexual experiences considered to be sexual abuse (showing/touching sex organs, kissing in a sexual way) may be seen as developmental rites of passage, part of a learning process (Widom and Morris 1997.)" Note that this “forgetting” does not mean that there is no psychological effect; only that the source of that effect is buried, becoming a silent trigger for self-destructive behavior.- http://www.genderratic.com/?p=836
 - Predictors of Sexual Coercion Against Women and Men: A Multilevel, Multinational Study of University Students (2007) - http://pubpages.unh.edu/~mas2/ID45-PR45.pdf
EDIT: Some clarifications taken from the feministing post of that survey
EDIT: Genderratic has a great post on that same survey. Read it here. Added one part of their article to the sources section.