Less than 24 hours after reading, "The Mac and Linux builds are currently in testing and will be available soon", the test game client is now available on Steam for Linux. I've heard from Phoronix readers (e.g.) that as of tonight they are seeing the Dota 2 test client available.
Dota 2 is the multiplayer online battle arena game sequel to Defense of the Ancients and under Windows has proven to be a highly popular title. Dota 2 is powered by Valve's Source Engine.