Keep in mind that having her boyfriend's semen wouldn't prove a whole lot other than the fact that they had sex recently- it is hardly mindblowing concept that she had sex with her boyfriend earlier in the day.
It could be relevant for the defense, especially if Nifong sticks with this theory that the players could have worn condoms. If the SANE exam showed vaginal injuries consistent with rape, and the defense could prove she had had sex very recently with her boyfriend, they could argue rough sex with (or even rape by) the boyfriend accounted for the vaginal injuries, raising reasonable doubt. As far as other men having sex with her, if she denies she had other clients that night it may be difficult for police to discover their identities in order to collect their DNA. It would be useful to know how long after sex semen could still be recovered from a woman in a DNA exam.
One of the attorneys was on TV yesterday with the information Nifong gave the judge in order to get a search warrant; in it he stated flatly that the DNA "would positively" identify the player, or words close to that.
Evidently they are now going to say that if he was that sure when he went for the DNA, he cannot not say that they might have worn condoms -- and that he cannot say that lack of a match doesn't mean anything.
I got the impression these guys are not going to take much crap off Nifong.