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