This was my first "year-round" commute year and currently I'm the only person at the office (a huge goverment agency) that rides a bike to work. Granted, we're in the suburbs and I have to combine the subway with biking, but still. When I worked at a major university last year, two men commuted occasionally, but I was the only person who rode a bike no matter what.

I think women are less likely to for the appearance reason. It's hard in corporate situations to look decent after riding a bike to work. It can be done, and I know some women who do, but it's a difficult lifestyle choice to make.