C2D MacBook w/ 2 gb RAM here. I run WoW with all of the settings minimized. I find the biggest performance hits come in the Outlands. Questing in the world is fine, but when you go to a largely populated area such as Shattrath City you can start to see some hiccoughs. The major hits, at least for me, come in instances in the Outlands and the higher end raids in Azeroth where there are many players. I pride myself on being able to do a good job in an instance/raid (warrior), and the choppiness makes this difficult to do. In sum, a MacBook is fine for doing world quests, but less than ideal for instances.
I don't know myself, but the age of the server that you play on might also have an impact on performace. I leave that determination to those much wiser and knowledgeable than I...