Generally I've found that if you're setup for home theater, you're also ideally setup for most games as well. I prefer to switch to stereo on Guitar Hero and Rock Band to try and limit the delay i get from my a/v processor. Metal Gear Solid 4 sounds pretty great in 7.1.
I haven't been quite as impressed with Killzone's sound as they seem to use a lot of fake sounding reverbs in their sound environments. Call of Duty World at War does a much better job as their sound renders to the environment directly in 3d. (so if you fire a weapon or blow up a grenade, you can hear that sound bounce around the environment.. back off buildings, etc.)
The new Chronicles of Riddick: Dark Athena also has really high quality sound, which I just picked up this week. I could probably build a more detailed list, but in off the top of my head here are some more that sound great: Flower, Frontline Fuels of War, BF: Bad Company, FEAR 2, Tomb Raider: Underworld, Resident Evil 5, Undertow (especially for LFE) and Legend of Spyro: Dawn of the Dragon (again big LFE).
Another game coming up soon with 7.1 sound if Prototype, which looks pretty fun.
