I think Valve should wait a year before porting Steam. The multimedia and audio frameworks and APIs on Linux are currently a mess. The APIs and frameworks need to be stablized before any sort of large scale professional game development can begin on Linux.

http://braid-game.com/news/?p=364

If Value ports Steam to Linux in its current state, I'm afraid it will become unprofitable and they'll quickly abandon it. If that happened, there would be virtually no professional Linux games for years.

Linux needs to be ready before we fall under the spotlight.