Right now the game they have been working on the most for their Linux client is Left 4 Dead 2. Why L4D2? Basically because it is a stable code-base to do for their initial porting. They have plans to bring their other titles to Linux as well as lobby other game developers using Steam and the Source Engine to bring their games to Linux -- natively. They are not using the Wine library or anything in terms of their native Linux Steam/Source engine work.
Check out the full article for more details.