[Since they’re porting Source][link], that means Half-Life 2, Left 4 Dead, Team Fortress 2, Portal, Portal 2 and all Source-based games will be available for the Mac. Jimminy.
[link]: http://www.wired.com/gamelife/2010/03/steam-mac/ “Valve Brings Hit Games, Steam Service to Mac | GameLife | Wired.com”