Our friends at Joystiq reported on this fascinating survey by GamesIndustry.com that broke down responses from 13,000 US and EU citizens about how they spend their gaming time. While they don't have specific numbers for World of Warcraft, the survey says [PDF link] that overall, 15% of gaming time is spent playing MMOs. US players spend about 14% of their gaming time on MMOs, while EU players range from 8% to 16% by country. In the EU, 14% of all players have played an MMO, and in the US, 21% of everyone playing games has played a massively multiplayer online game.
Again, these numbers aren't specifically for World of Warcraft (and because the survey went down to age 8 and up, they do include the vast virtual worlds aimed at children, like Club Penguin and Disney's Toontown, which probably throw the numbers off quite a bit), but they do show the effect that WoW has had on the gaming population over the last five years. Five years ago, MMOs were definitely a niche -- some hardcore gamers played them, but most people didn't have the Internet connections to play an online game, much less pay a subscription for one. Nowadays, MMOs represent over one out of every ten minutes of overall playtime, and those numbers are only going to go up.