A Steam client for OSX is looking more and more likely now and this is great news as far as I'm concerned. About time the Mac got a better game delivery platform and it means I wouldn't have to boot into Windows nearly as often 🙂 As to the games themselves, I can only echo what others have said: Most titles would still need to be ported but, several already have been so 20%? 30% of the catalog *could be available right from the start. Source engine games ported to Open GL/OSX would kick major ass too! Half Life 2 and Counterstrike:Source are amazing but I'm salivating over the possibility of playing... Left4Dead, Left4Dead2, Portal, Day of Defeat and a hundred others under OSX.
Bring it on Valve... please