Here's an interesting bit of info from the Nielsen folks: over 400,000 women are playing World of Warcraft in the US, which means it's the most-played "core" game for that gender. And even more interesting, females 25 years or older make up the largest block of PC game players overall, and they account for 54.6% of all gameplay minutes in December of last year. Girls don't just play WoW -- they're quickly becoming one of it's main demographics.
You can read the report in PDF form over here -- the chart above might be the most interesting piece of information, as it shows that though males still make up a huge part of the PC gaming audience, many of them have now moved on to consoles, and women (especially older women, over 25), during the last month of last year, are making up a huge audience for PC games. Later in the report, you can see what kinds of games women are really playing: Solitare, Freecell, Minesweeper, and all of those other little attention grabbers on every PC. But among those widespread casual games is our own World of Warcraft. And while the 25-52 male audience of 675, 713 for that game still remains larger than the female audience in the same demo, the ladies aren't far behind.
Neilsen also calculated some base stats for WoW, including the fact that 1.8 million unique people played the game, and the average time of gameplay per week was 744 minutes, just over 12 hours (slightly up from last year's average). Additionally, of those who play World of Warcraft, their second most-played game was Solitaire, followed by Warcraft III. Fascinating stuff. Remember that these are statistics, so they are more general trends than anything else, but it's definitely true World of Warcraft and PC gaming in general is no longer only the domain of the male demographic.