Steam’d Penguins on the Valve Website has new details on Valve's progress in adding Linux support to Steam and their games. Here's word on their progress on a Linux Steam client and a Linux edition of Left 4 Dead 2:

The goal of the Steam client project is a fully-featured Steam client running on Ubuntu 12.04. We’ve made good progress this year and now have the Steam client running on Ubuntu with all major features available. We’re still giving attention and effort to minor features but it’s a good experience at the moment. In the near future, we will be setting up an internal beta focusing on the auto-update experience and compatibility testing.

Since the Steam client isn’t much without a game, we’re also porting L4D2 to Ubuntu. This tests the game-related features of the Steam client, in addition to L4D2 gameplay on Ubuntu. Over the last few months, excellent progress has been made on several fronts and it now runs natively on Ubuntu 12.04. We’re working hard to improve the performance and have made good progress (more on that in a future post). Our goal is to have L4D2 performing under Linux as well as it performs under Windows.