I've been playing wow on my 15" G4 powerbook for the past 6 months or so, and I'm fairly happy with it.

It's a 1.25GHz G4, with a mobility 9600 and a gig of memory. I also have a reasonably powerful desktop PC which has enough oomph to play it at 1280 at maximum detail, so I have been able to compare.

A few things I've found about wow on a mac (or more precisely, a PB):

1. You need a gig of memory, it just chugs and chugs on 512mb.

2. Terrain draw ahead distance has the biggest impact on fps. I have it down at the lowest, and virtually everything else on.

3. I run mine in a window (so I can flick back and forth between wow, adium and thottbot), at about 1024x768 (or is it 800x600?). So I'm not running at a high resolution.

4. Ironforge is excruciatingly painfully slow around the auction house.

With the above combination I average somewhere between 15fps and 25fps, which is much better than I had hoped. I find it very playable.