Ok, this a matter of extreme debate in some circles, but I would imagine alot of it actually has to do with brain wiring as well as social upbrining. It's probably not so much a matter of women not in the programming field, but personality types. If you took an across the board look at programmers, I'd be willing to bet you'd see a strong showing of a particular personality type. From what I've heard/read from social studies and personality studies (propoganda?) it tends to go that women tend to be more creative and social, and men tend to be more analytical and less social. Computers, for quite a while before the boom of the internet, have been the domain of the analytical and mostly non-social. Couple that with the strong social notion (though it's fading more and more) that women "aren't supposed to be technical" (i've heard the quote more than once). Now, I'm a US citizen and this is coming from that perspective, but i'd also be willing to bet that you'd find decent differences in different cultures on the amount of women to men programmer ratio.. There is a lot that goes into this question.. and there is no easy answer.. but the playing field will eventually even out, possibly even before our lust for technology does some real damage to us..
morbid eh?
-Syn0

(/me wonders if i should have really posted this.. such a touchy subject)
Updated: reworded some things :)