A recent update to the Team Fortress 2 beta client suggests that Valve is adding Linux support to the game.
As spotted by an eagle-eyed Slashdot reader, the list of changes in the Oct. 22 patch includes Linux-related files such as "a hardware driver compatibility list, optimal graphics settings, and a shell script launcher (previously only for OS X, now with a case for Linux as well)."
Valve's official changelog for the Team Fortress 2 beta makes no mention of Linux; it only says the patch provided "performance improvements" and brought the beta client in sync with recent changes to the game.
This summer, Valve announced on its new Valve Linux blog that it was working on bringing its Source engine and Steam to Linux. Developers were working on porting Left 4 Dead 2 to the Ubuntu distribution of Linux, with plans to make it the first Valve title to support Linux. An update from Valve last month said the company would hold a private external beta for 1,000 users, with Steam and "one Valve game" running on Ubuntu, in October. However, Valve has given no indication since on when that might occur.
We've reached out to Valve to ask about Team Fortress 2 on Linux, and will update this article with any information we receive.