I'm not comparing my PC to my mac.
The half perf diff is if you compare a mac to a comparable PC.
Like if I compared my PB to my kid's 1.26ghz P3 PC with GF4MX. That'd be more comparable, but the PB will probably still be slower for WoW than my kid's.
PB's aren't going to do much for WoW, but they are playable. Maybe after a bunch of updates, it'll be on more equal terms, but for now, you'll have to turn a bunch of settings down. I've seen bench marks of the 15 and 17" PB's that don't look much better than the 12" models.
Basically if you want pretty gaming, you may be better off with a PC of equal pricing. If you want a better laptop with less issues, PB's are good to go. WoW will play, but i wont look as nice as on an equal priced PC.