Valve will be porting Steam to Linux with full functionality. The objective of the Steam client project is a fully-featured Steam client running on Ubuntu 12.04. Valve now have the Steam client running on Ubuntu with all major features available. In the near future, they will be setting up an internal beta focusing on the auto-update experience and compatibility testing.
Steam will also be 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, Steam say excellent progress has been made on several fronts and it now runs natively on Ubuntu 12.04. Steam are working hard to improve the performance and have made good progress and their goal is to have L4D2 performing under Linux as well as it performs under Windows.
Via Valve Software