Honestly, trying to figure out why there are a lot of women who never will be interested isn't really a priority for me at the moment. If we look at *open source hackers* as a slice of the population of the entire world, we're a minority - no question. So addressing the issue of why more *people* aren't interested in hacking on open source -- I think the same reasons apply to women.
The stuff that Nat Torkington (and many others) have talked about and done -- volunteering at schools, and finding ways of integrating interesting/fun technology into curriculum, and starting very early (primary school) -- are important. But those aren't the only ways that we can change our culture. We can actually change how many women are involved *now*, by simply looking around for the people who are on the fence.
I think it is counterproductive for hackers to throw up their hands and say, "Well, most women just aren't interested" when the due diligence has not been paid to encourage people who are interested, but not participating -- for whatever reason.