I used to do this back-of-the-napkin style presentation on whatever paper was handy when someone told me in person that women just weren't good at math, and that's why there were so few women in computer science. I'm not sure what possesses people to say stuff like that to female mathematicians, really.
Anyhow, the point is that yes, there is some gender disparity in math skills, but if you do the math, it simply doesn't add up: those differences simply cannot explain why there are so few women in computer science (or in open source software, or in physics, or whatever).
Many people misunderstand or abuse the information we have about gender and ability differences. Hopefully this presentation will explain what the data really says (which isn't very much).