Hopefully id get all their old games up on that list soon, they put in a lot of effort over the years when not many others were, a shame that changed recently though.
I'm interested in TF2 and Red Orchestra but look forward to my old favs...HL and HL2! I might even buy a new system so I can play the whole L4D series too!
My understanding is that the plan is for all Valve Source Engine games are going to be ported (and should be relatively simple now that TF2 is working).
Everything else is up to the various game studios, though all games in the Humble Indie Bundles support Linux so hopefully they are added to Steam soon.
uplink works. defcon doesn't, unless you some manual "patching" (download the demo, fire up defcon in wine/windows to get your steam key, activate it on linux). It'll eventually come though.
My impression was that they started the project by porting Left4Dead, but decided to release TF2 (probably because it's F2P and so anyone can test it).
Very few games are compatible, but Team Fortress 2 is one of them.