Okansas links to an interesting study in the Scientific American which relates the cycling friendliness of a city to the male-female ratio of the cyclists in it – the theory being that men are more likely to brave a motor-friendly place while women need more encouragement.
I counted 19 men and 17 women on bikes on my commute into work today, although this was after the normal London commuting time, and a significant part of the commute was not on roads. I suspect that there is more of a male bias on the busier roads and during the rush hour.