There's also a fantastic (and lengthy) profile of him in the current issue of Rolling Stone, which unfortunately does not appear to be published online.
I'm going to ignore the naseau coming up from the craft comments, and state that I think it's entirely a matter of numbers.
If one in a thousand programmers contributes to open source, and 28% of those earning computing degrees are women, and 50% of them remain in computing careers, we certainly should expect very few women in open source.
Add to the fact that women in families still do most of the housework and childrearing, and optional fun activities like contributing to open source are going to happen in even smaller percentages.