Just announced, and hot off the press! It looks like Valve has completed porting the Source engine to Linux. TF2 is now the first source-based game to be officially released for Linux. Value is encouraging people to download and try Ubuntu, stating that it is "our favorite version of the Linux operating system", and asking users to "see what you think".
I feel like trying it, just to see if Valve's claims of better gaming performance on Linux, as compared to Windows, is true (can someone try it and post their results here?). I get the feeling that this is only the beginning of Valve's increased emphasis on Linux.