Well Portal will be free, as their using it to advertise for Portal 2. TF2 and of course HL and HL2. I think I read that LFD2 will be too, not sure about that though. I don't know what to expect going forward, it's not like Valve is opening a new market to these publishers since the games could have been ported over before Steam for Mac and sold outside of Steam.
I signed up for the beta but wasn't selected, sadface.
I've heard the Steam for Linux rumor, it's been going around for a long time hasn't it?