First, I would like to point your attention to the Battle of the Sexes, where topics like this usually go. But I guess this is OT as well, so I'll shut my big trap.
Umm, my girlfriend started masterbating since we have been dating (supposedly) and it seems that her sex-drive is up quite a bit. In fact, she wants it more often that I do (blieve it or not). I have heard a few times that when a girl used a vibrator, it desensatizes her, and that no penis could compare to a gyrating schlong. Anyway, I hope that isn't true.
Like Juvenile said, I would try to play into it. Try to be honest with her, but realize that you can't make her stop. Try to get her to incorporate it into foreplay or just find out what's going through her mind when she does it (might help you out during the real thing, if she likes it a certain way or something).
So I guess the only thing to worry about is if she's thinking of another dude while doing it. But no worries chap. Good times
-Ducky