I have installed Ubuntu Linux 4+ years ago, basically when its multimedia capabilities started to work in a fresh installation. I have never felt any necessity to come back to OS X/Windows since then.
Once Steam comes to Linux, I don't see many reasons why a lot of gamers won't switch over. Many of the people I who use Windows to develop in Python/Ruby/etc use either a VM or SSH into a Linux box, simply because they want to game. I think many people will not want to pay the Microsoft tax, especially for Windows 8.
Because most games on Steam will still only run on Windows. Until game developers take up the charge (which Steam on Linux might encourage), it's just a window (no pun intended) into what could be.
The only games that will be on GNU/Linux, at least at first are Valve games and crappy indie flash games. While it's the first step, most developers aren't on board. Yet.
Most of the Humble Indie Bundle games seem to be available on Linux and I wouldn't call them crappy nor flash. But yeah, the big titles rarely work. Valve's work on porting their Source engine to Linux will certainly help this.
