I have the 13" Core i7 MBP, so GPU clock speed is slightly higher than the base version, performance increase should be quite marginal though.
I only run Macos also, no windows at all on this machine. This limits gaming options quite substantially, so I've only tested Valve games, WoW and Torchlight. Half-Life Ep. 2 is a little sluggish, but runs alright I'd say. Obviously not as pretty as on a desktop, due to the low screen res and dumbed-down settings, but it's playable enough in single-player campaign.
TF2 is not as successful, on default settings it's fairly choppy for a competitive "twitch reaction" player-vs-player game at 800*600 pixels. Dumbing it down some more helps, but I've been having huge issues with mousing, due to pointer acceleration totally screwing up my aim.
Original Portal runs great. Portal 2...runs.

It's certainly not great, or even good really, but since it's not an action game you can get by. Hard to play with the Magic Mouse though if you're like me and want to jump with the right mouse button... Sometimes it REALLY slows down, particularly when inching up close to a portal.
WoW Cataclysm runs surprisingly well at native screen resolution (1200*800), as long as view distance is kept under control. Most of the advanced graphical effects aren't even available (either they don't work with the Intel graphics processor, or the Mac client doesn't support them period). I also turned down water and shadows to its lowest setting, and most everything else on low as well. Performance is good enough to hanker by on when just doing daily quests and such. I didn't try doing any dungeons or raids before my gametime expired. Haven't bothered to reactivate either.
Torchlight probably is the nicest game to play of those I've tried so far. As long as there isn't too much water on-screen it runs a smooth 60 frames/sec much of the time, even with every setting on max and anti-aliasing activated. I also think it's a rather fun game!
Diablo 2 doesn't work in snow leopard unfortunately, otherwise that one would have been ideal to play methinks.

Haven't tried Warcraft 3, but its graphics is so simple I'm sure it'll run great. Starcraft 2 is probably much like WoW; need to knock everything way down; haven't tried either even though I have it installed.