During the dev panel a few minutes ago, Tom Chilton told us something interesting about playing World of Warcraft on the Linux platform -- Blizzard has actually had it working. For "compatibility purposes," they apparently had an internal build of the game that worked on Linux. But unfortunately for Linux users, they have no plans at all to bring the final game out on the system. They didn't expound much more than that, but of course we can imagine all kinds of reasons they'd hesitate to release a Linux version, first and foremost being that in terms of gamers on the platform, the marketshare just isn't there.
Of course, this doesn't mean you can't ever play WoW on Linux, it just means you can't do so off the shelf. WoWWiki has some really great information on how to play WoW with WINE (which just, by the way, reached a 1.1.0 release, and allows you to play Windows programs on other platforms). Transgaming's Cedega is also supposed to work with World of Warcraft (and you probably won't get banned), and Codeweaver's CrossOver also is approved to work with World of Warcraft on Linux.
So there are ways to play it, but you might have to do some tweaking. Still, we wish they'd told us more about why they don't want to release the client they builtt