The news is a few hours old now: Valve recently distributed mock ads that, if anything else, implies that Steam will be available for Mac OS X.
Valve’s Steam is a popular online store for games. Customers browse the catalog of popular PC games—which includes recent hits like Modern Warfare 2 to DOS classics like Indiana Jones and the Fate of Atlantis—through a browser or the Steam desktop client. Games are playable once paid for with a credit card or PayPal, and after the desktop client has downloaded the content. No need to keep a game disc in your optical drive, no need to jump through copy-protection hoops.
Throughout it’s entire existence, Steam has been Windows-exclusive. Linux enthusiasts have gotten some of the games to work on their favorite build, and of course Mac users get BootCamp to run Windows on their computers. But Valve’s lucrative gaming ecosystem has been officially a Windows PC project.
Bringing Steam to Mac OS X involves certain obstacles, from a development point of view. Most of the games available on the catalog don’t have a Mac version, especially older games that originally ran on DOS. But since Apple’s PCs use the same hardware as Windows PCs, I don’t think Valve will have too much trouble, especially if Apple has started taking Valve seriously. A good number of games already have Mac counterparts/work on a Mac anyway.