I am an active duty AF doc and agree with most everything efex says about this. BTW, it's not only citizens, perm residents, but even nonresident (e.g. illegal) aliens are expected to enroll with the SS! Right...
Anyway, I think if there ever becomes a danger of having a draft again possibly there will be something new about females being included for "fairness' sake." However, doing something like this with the registration process right now would really be more for show than anything, because in the military the primary concern is not the right of the individual (which is really what this issue is about, in my mind). The primary concern is always the mission, and whatever policy changes are made are in lieu of this.