It looks like if you're part of the Steam for Linux group, Valve is slowly starting to add you to their beta. There aren't a lot of games available (~27), but they're adding new ones fairly regularly. Don't expect to see any major titles like Borderlands any time soon, though. Unless you're exceptionally good at convincing developers to spend some money porting their game over, it probably won't happen.
So far, it looks like it's mostly indie titles on Steam for Linux. Some notable games:
- Amnesia
- Limbo
- Osmos
- Serious Sam 3: BFE
- Solar 2
- Team Fortress 2
- Trine 2
And a few more! I've read some rumors that seem to suggest Portal/Portal 2 may be released within a month for Linux.
