Portal 2 coming to the Mac?

While I'm about as excited as everyone else for the next Portal, I might be even more now that I've heard it's coming to the Mac platform.

Yes, that's right, if you haven't heard it yet, and you get all of your news from PLAY READ WRITE (who are you?), Portal 2 is coming to the Mac OS. While this is great news for Portal, especially because any extension of the audience for that game is only going to translate to more sales, I think I have bigger questions on what this may mean for Valve games (and PC games) and the way people play them.

My first question is: will I be able to play TF2 and other source titles on the Mac now? That's a big question to jump into I know, but we already know that a version of Steam is going to be available to the Mac platform already. Of course, my next question from there is, will my current library of Steam titles be transferable to the Mac platform, or will I have to purchase Mac versions of all my games?

I may be moving a mile a minute here, but all of these are very valid thoughts. Where is Portal 2 going to lead the rest of the Valve library and the Steam store in general? With soon to be competitor OnLive supposedly allowing cross-platform play of so many titles through streaming, wouldn't Valve and Steam want to expand their reach and value proposition a little bit?