For as many bike commuters there are around here, I'm surprised that I've only seen 2 women that look to be commuting (panniers or backpacks). I encounter over 25 commuters on my round-trip, so it's < 10%. There are a few other women that I see riding, but they don't have any typical commuter gear and I only see them 1 or two days a week if the weather is nice. Maybe they're commuting, but I can't be sure.
I know the imbalance isn't for a lack of female riders around here, because there are plenty of women riding in the organized event rides.