Yay! Valve officially announced Steam and Source for Mac yesterday! Steam is like Xbox LIVE for PC gamers. Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal, and the Half-Life series will be available via Steam on the Mac next month. Best of all, Valve's titles will be using their new Steam Play feature so anyone who purchased the PC versions can download the Mac versions at no additional charge! Starting with Portal 2, future Valve games will simultaneously debut on both Mac and PC.
For game developers, Source now works natively on the Mac, making OS X an easy target for studios who use Valve's top tier game engine. Steamworks lets studios self-publish to the Steam community of over 25 million online gamers—a number that's sure to grow with the addition of affluent Mac players. Steamworks publishers include practically every major video game company who now have more incentive than ever to make Mac versions of their games. One wonders if a version of Source and Steam for the iPhone OS is in the works...

