Steam have announced
more details about the Mac OS X version of Steam, coming soon:
The first collection of Mac titles will demonstrate "Steam Play," which allows customers to purchase a game once and play it on all Steam supported platforms. Gamers who have previously purchased games on Steam will find them available in their account on the Mac as they are released.
promoted/edited forum item
Portal, in addition to supporting Steam Play, will be the first of Valve's Source engine based games available on the Mac. Native OS/X support for the Source engine is also available immediately to licensees for use in their games.
On subsequent Wednesdays, additional collections of Mac titles will become available, each designed to highlight specific functionalities of Steam on the Mac.