I have been playing it for years on a 1Ghz G4 and I game with people playing WoW on a 800MHz G4. I was getting between 7 and 14 fps (7 fps in Dalaran) with the settings minimized so it's just a matter of what you consider playable.
I just picked up a quad 2.93 Mac Pro and playing WoW is much more enjoyable .
If you wanted to know it runs perfectly and it never has sketchy frame rate or anything even with the settings on full. It did take like 16 hours to download though.