I myself have the MacBook Pro 15" with the 256MB graphics Card option (2 gigs of RAM as well) and I will say that i'm quite confused with the consistancy of gaming performance on the Macbook Pro via BootCamp. Though I assume that it's just because it's in beta right now.
Half Life 2 runs fine, with every setting turned up to full, except for The Anti-Aliasing which is turned off. Counter Strike: Source, however, hiccups randomly during gameplay. It gets distracting, and quite annoying during the middle of gunfights.
Halo runs smoothly, but for some reason looks a little goofy. Doesn't look quite as solid as I expected it would, especially with maxed out settings.
Doom 3 runs pretty well in High Quality resolutions, without any third party optimizers enabled. Only every once and a while do I notice a 'click' pause, but it's over pretty quickly so it doesn't get in the way too much.
Defcon runs without a hitch, as does Darwinia.
Another World Collectors Editon runs smoothly and beautifully, benifiting a native support of 1440x900.
Phantasy Star Universe has some severe slowdown problems in certain maps, which I experienced even on my main PC.
Silent Hill 2 has a recurring sound error that the patch did NOT fix.
Silent Hill 3 has horrible slowdown issues (that were not present on my PC) and has a refresh rate problem which gets very distracting during combat situations.
That about sums up everything i've tried on it. Strangely, most of the problems that arise are telling me that the X1600 in my MBP is WORSE than my old GeForceFX 5200. Which was considered mid-range back in 2002.