Valve Announced that Steam and Source support for the Mac.
Steam and Valve’s lineup of games including Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal, and the Half-Life series will be available to Mac users this April, but from here on out, simultaneous cross-platform releases should be the normal.
Portal 2 will be the first game to launch simultaneously on both the PC and the Mac. But don’t get confuses, Portal 2 is still not coming out until the holiday season. It will just be the first game to release on the PC and Mac at the same exact time. All the other games are already on the PC so they cannot launch simultaneously.