Prevention of pregnancy: Although pregnancy caused by rape is rare (except in the few days before ovulation), emergency contraception (see Family Planning: Emergency Contraception) should be offered to all women with a negative pregnancy test
I did some cursory reading on the subject.
The evidence on the chances of conception of rape seems to be mixed. Some say rape lessens the chance of conception, some say it increases chance of conception (because exposure to semen induces ovulation) and some say the chances are even.
I have a hunch there's no conclusion that can be drawn "all things being equal" because of the numerous factors involved: biochemistry, birth control, "types" of rape (forcible, versus statutory, versus date rape, versus drug rape versus blackmail versus.. on and on).
I don't think Todd Akin chose the correct word when he said "legitimate" but I think he was trying to contrast between true and false accusations of rape. Yes, women do make them.
UPDATE: You might want to read Kunoichi's blogpost on the subject.