Clay Shirky has written about this at length [1]; the basic point being that women are socialized their whole lives to not be entitled or aggressive but to stay quiet and in the background, while men are constantly socialized to take what they want, step forward, go for it, "be a man" etc. Obviously there is much variation, and yes, geeks tend toward the not-so-aggro end of the table, but I think it's still an important point.
I'd also suggest that, sure, there are lots of men not speaking at conferences, but I suspect proportionally there are even more women for whom that is true.
[1] http://www.shirky.com/weblog/2010/01/a-rant-about-women/
(edit: added link)