It seems pretty obvious that Valve is bringing Half-Life, Portal, and Team Fortress 2 to the Mac. The big question is, will they pull it off. Imagine if Valve could bring, in one foul swoop, all the games on Steam to the Mac. Let's be honest, gaming on the Mac is restricted to a few titles, Wow, iD software games, and a few others that no one plays, except to say "but there is gaming on the mac!"
If suddenly Valve had all the games on Steam on the Mac, with a tried and true delivery platform, that would be huge.