From everything I've seen in my time, it's simply a case of women I know not wanting to program.
There seems to be a general bias in the job fields between the genders, with a larger quantity of women seeming to choose jobs in the 'caring professions', such as medical and nursing, communications (linguists), arts business and the more socially interacting disciplines.
I've met a few women coders along the way, and those I've met tended to be not really any different to the guys.. Some were awesome coders, some were reasonable.
It's definately not a case of aptitute, and (in most cases), it doesn't seem to be a case of a big preference for men in the jobs.
From my personal experiences, it's been a case that the women I know have chosen to follow other careers that they feel better suited to them personally.
One of my old girlfriends had quite a response to the whole issue, which was that she was happy doing what she chose to do, and would guys feel happy if women constantly needled them that they should consider jobs in nursing, because there were too few men in the field?
If women choose to code, for the pure reason they feel like doing it, then good on them.
If they don't choose to, for the same reasons, then, who am I to start saying that they should, just because there aren't very many of them in the field?
Like you mention, Cybercosis, it seems that a lot of the dearth of female coders is simply down to personal choice.
Perhaps in the future, that may change, as subjective perceptions change with the social trends prevalent in the days..
Maybe it's just guys are more antisocial.

Anyhow, that's just my take on it. :)

Cheers,

Malk