The word is that Caster Semenya is definitely "intersex", so she was born with both male and female characteristics. Now what I can't confirm, but seems to be the rumor is that she has no uterus or ovaries, but she does have undescended testes. Not sure how true that is, but she does naturally have way higher testosterone than most women.
I think the same may also apply to one or both of the other women that medaled in the women's 800. Not as far as being intersex, but supposedly, their test is also exceptionally high.
Makes for an interesting discussion.