I heard about this the other day, and I'm surprised it hasn't been mentioned here yet (trust me, I looked). Anyway, ComiXology, "cloud-based multiplatform digital comics reader," recently did a survey of 16,000(!) readers, and they found that an increasing number of customers are women. In fact, they also found that just within the three months prior to the survey, 20% of new customers are female. There isn't any reasoning provided about why there are more lady customers, but you can read more about the survey here.
Anyway, I thought this survey was really interesting. As a woman, I've had a weird relationship with comic books and graphic novels. My brother is ~9 years older than me, and he had an extensive comic book collection, and he never ever shared them with me (although I used to steal his binder full of X-Men trading cards and make up stories about them). And there's a tiny part of me that wonders if he would have shared his comics with me if I were male. At any rate, I've always been a bit of a geek, but it's only very recently that I've become interested in comic books, and I'm sure it's been helped more than a little bit by my participation in the O-Deck and Groupthink (heyyy FightinginFishnets). I really hope more women do read comic books because I think it's a medium that has a lot of potential, plus superheroes and whatnot are just plain fun.