I haven't read all these posts, but want to include my two cents:
Frankly, I grew up not realizing that "girls don't game;" I know a lot of female gamers in real life. But, because of this, I better understand something: It isn't that girls don't game, just that in many cases, they don't WANT people to know they game because it makes others feel awkward.
This isn't a problem among our friends, but it becomes one among strangers. One gamer girl I know works at Gamestop...And she somewhat regrets it. This is because she gets one of several responses from guys walking up to the counter:
1. The guy is socially awkward and nervous in the presence of one with boobs and a uterus.
2. The guy starts flirting with lines like "Hey, you want to come to my place and play some Halo? We could do it ALL NIGHT LONG!"
3. The guy says something along the lines of "Why do you work here? You're a chick, you don't play games!"
The first and third ones are most common. I went to a game store once with a few gamer girls I know. One started playing a game on the demo 360 (It was Call of Duty 2; this was shortly after the 360's launch). She stopped at one point and looked around. There was a ring of guys around her watching, slack-jawed and sweaty, like (a) the concept of a woman playing a shooter was strange and near blasphemous, and (b) they wanted to talk to her, but were all too scared...Which is why they were keeping a minimal seven foot radius. Finally she said "What, are you just going to gawk at me all day or say something?" and they all looked even more nervous, with several taking steps away from her. I kid you not, this actually happened. Frankly, the girls I know somewhat dread going into a game store because they ALWAYS get these responses.
Now, the reason why girls don't play on Live very much is because, well...You guys all know how terrible the community is WITHOUT women, imagine what it is like WITH girls in the equation. The racist homophobic pre-teens become HORNY racist homophobic pre-teens. I remember the one time my girlfriend joined our party in Call of Duty 4. She talked and a screechy-voiced harpy of a 13 year old yells "OMG A GIRL!!!! YOU'RE A GIRL?!!??! ARE YOU HOT?!?! DO YOU HAVE PICTURES OF YOURSELF?!!? IF SO, I WANT TO FUCK YOUR PUSSY UNTIL YOUR NIPPLES SHOOT MILK ALL OVER THE PLACE!!!!!" That was the last time she ever talked while we were playing public matches. Another girl I actually met through Live that knew some of my friends gets verbally raped on a daily basis when playing online. It is truly sad and pathetic. That, or you get the jerks that view women as a lesser species. In fact, the first time I ever played when she was in the party we had to deal with one of the "women suck and exist only for me to rape" assholes.
So, in a way, part of the reason why girls don't game as much as guys (And why many of those who are hardcore gamers keep it a secret) is because of how they are generally treated by guy gamers. If they aren't avoided by socially awkward gamer geeks, they are hit on in the most vulgar of manners, if they aren't being hit on, they are being interrogated so the guy knows that she actually IS a gamer, if not that, then they are just generally being mistreated by the more sexist gamers. Really, if they weren't treated so negatively, I'm sure that girls WOULD play games more often.
Secondly, most games are aimed solely at guys. Sure, a lot of the girls that do play games are playing the same games as the guys are, but that doesn't change the fact that SO MANY are overwhelmingly masculine. Games like Gears of War and Halo, popular mainstream games, just ooze out testosterone. There is little there for many girls to like. And so many games try to copy these games by having "tough" super-muscled space marines in enormous hulking suits of armor.
When games aren't oozing testosterone from every orifice, they treat women as sexual objects. I mean, sex appeal and having some really sexy female characters is okay here and there...But in gaming, this is almost ALL developers do. Soul Calibur, DOA, and countless other games include female characters with GG breasts barely contained within the bare minimal leather dominatrix suits they are running around in. Really, I don't think too many girls are interested in having to stare at the g-string burrowing into the ass of some sex-deprived concept artist's sexual fantasy for hours at a time.
That said, the girl gamers out there do play the same stuff we guys do, but many are more drawn to more niche titles that aren't overwhelmingly popular with guys. It isn't a niche title, but we all know how The Sims is really popular with female gamers. For every gamer girl that likes blowing off heads in a shooter, many others rather play less violent games like The Sims. So, a guy who only plays Call of Duty 4 or Gears of War on Live probably won't see too many girls playing. As I've noticed, gamer girls are often more drawn to strong stories, high levels of customization, and games that let you have a lot of choices in what you do. Several of the gamer girls I know are really interested in strategy games; my girlfriend's little sister really likes Supreme Commander and we recently got her playing some of the 4X games like Sins of a Solar Empire. And of the gamer girls I know, ALL of them like RPGs to some degree, and play them mostly for the story and characters. That said, I know several that like shooters too.
Really, it isn't that girls are inherently not interested in gaming, it is just that there are several barriers that prevent girls from playing games. Those barriers are primarily the fact that the game industry has mostly forgotten the potential in female gamers...And the ones that haven't confuse WOMAN and LITTLE GIRL, cranking out crap like Barbie and kitty cats and puppies and such because they think it appeals to ALL female gamers...When instead, it is just insulting. The other barrier is the general view of women within the game industry and gaming community.
But, at the end of the day, you'd be surprised just how many women actually are playing games.