The Source (and Goldsrc) engines support a Linux dedicated server, which explains the job listing. As far as using Linux goes, unlikely, since people won't be able to play any of the games they already own.
Haven't they already ported most of their games to mac? It doesn't seem like going to Linux would be that big an effort. Valve's games make up like 10% of steam sales though so they would need to work on pressuring the other 90% of the market that they don't control. Releasing a dedicated box that doesn't work with 90% of the content you have sold doesn't sound like a winner to me.
Not entirely true. Valve's Source games can be easily ported, and other games can be modified slightly to run on WINE. (Similarly how all of EA's Mac games run on WINE instead of being ported)
