Virtually all the comments on here are missing a key point: everything they say is true, but it was true twenty years ago when there were a lot more women going into computer science than are now, though still far fewer than men.
I think what's changed is that many other professions have openeed up to women. When I started work as a programmer, almost all doctors, lawyers, etc. were male. Computer programming, though, was much closer to a pure meritocracy - no one cared about your chromosomes so long as you could code. So it was a good avenue for intelligent women to pursue. Now, they have plenty of alternatives and, as others have noted, women seem to be more inclined than men to pursue more people-oriented professions, like doctors and lawyers, when those options are open to them.
Somebody ought to cross ball point pens with coat hangers so that the pens will multiply instead of disappear.