With recent chatter from Valve about Steam coming to linux system eventually, it looks like its going to be a reality sooner rather than later!
The Left 4 Dead 2 game blog post mentioned this:
"All of this will also be available not just on the PC and Mac, but also for Linux users as well starting in the middle of next month and rolling out from there. We will have more details as we get closer to launch."
Purely awesome news for Ubuntu linux users. More games make Ubuntu and linux a viable operating system for millions more users around the globe.