This whole thread is very interesting. I've been experiencing something similar to the issue described above when playing WoW for an extended period. My own experience leads me to believe it's a heat issue, but hey, I'm no computer whiz.
I do experience a significant drop in performance after playing for an hour or so: The laptop is pretty hot on the bottom, but not terribly hot, so I'll usually plop a remote underneath. I've tried lowering a few of the video settings down a notch and that helps tremendously. I believe lowering the 'ground clutter' option yielded a significant performance increase. I would much rather not decrease my video settings, and I'm not suggesting that as an ideal, but it's a solution which, for the time being, has worked for me.
I generally run with all settings maxed: Shadow detail set to 'Low'; glow screen on or off, depending on my mood, with no significant increase or decrease in performance. Usually my fps stays between 40-120, never lower than 40. When this issue hits it will drop to around +/-25fps, going up and down, depending on what is drawing on-screen. High forest areas tend to have the most impact on performance when this is happening.
The next login, after I've stopped playing and the computer has had a chance to cool down, everything is back to normal until the laptop heats up again. This leads me to believe the issue I'm facing is more attributed to a GPU overheat than faulty soft/hard-ware.
I run WoW in OSX. I wonder if persons running in Windows have similar, or worse, issues.
Regarding the above quote: Do remember that the WoW graphic interface received a face-lift with the arrival of WotLK, as well as the patch preceding it.