Linux use now comes in at about 2%, which is nearly double that from one month prior. Ubuntu 12.10 x86_64 is the most popular Linux distribution followed by other flavors of Ubuntu, and Linux Mint now comes in at just over 0.1%.
While Linux gamers won't be taking the majority market-share away from Microsoft Windows anytime soon, the current Apple OS X usage is about 3%. Prior to Valve introducing their Linux-based Steam console, I wouldn't expect the Linux market-share for this survey to rise above 5~6%.
Details in full for the Valve Steam Hardware Survey are available at SteamPowered.com.