Makes me wonder if this is too little, too late. Anyone who is an avid steam user already has a Windows partition for their games, and probably has a large library of games that will not work in Linux.
It's great that Valve games will work on Linux, and it'll likely unify the Linux gaming experience, but Windows is the #1 OS for PC gaming and that isn't going to change anytime soon.
Heck, look at OS X. They've been a gaming-capable platform for far longer than Linux and they're still in single-digit Steam adoption rates.
It's a positive feedback loop - gamers choose Windows because developers support it the most, and developers support Windows the most because there are more gamers on it.
Nope, I think Steam will be the best thing to happen to gaming on Linux. Ostensibly, people will get Linux versions of their games (even their old games) for free (didn't they do that with the Mac version?), and developers can get up-to-date statistics of how many Linux users they have exactly.
I know I sure would like Steam on Linux so I can stop having to reboot to play a short game. I'd play quite a bit more if Steam were available.
Video games are the only reason I even have a Windows partition.