This freezing problem is somewhat of a known issue with the latest generation iMacs. I don't play WoW, but I have read of identical things happening (and a possible solution). I have the same problem, only with UT2004.
The good news is, it appears to be a software/driver problem with the 2600pro/xt graphics card. In which case, Apple will (or had better!) fix it in a forthcoming software update.
A post I read on the Apple discussion forums suggested pixel shaders are to blame. And I believe WoW has an option in the game settings to turn pixel shader effects off? In which case, try that, and the problem should go away.
Also, does this happen in fullscreen and windowed mode? Try changing it. This may also help in WoW.
Unfortunately, my iMacs freezing issue with UT2004 is harder to fix, because there is no simple "pixel shader effects" check box to turn off. So unless I crank absolutely everything down to the bare minimum of settings, it will freeze at some random point a few minutes into a game. So its a process of trial and error to see what i can enable again without it freezing up.
In the mean time, it seems like less hassle to make do with playing it on my old G5 iMac and wait for an update to the new iMac to fix the problem.
Good luck!