We are treating the Mac as a tier-1 platform so all of our future games will release simultaneously on Windows, Mac, and the Xbox 360. Updates for the Mac will be available simultaneously with the Windows updates. Furthermore, Mac and Windows players will be part of the same multiplayer universe, sharing servers, lobbies, and so forth.

John Cook

Left 4 Dead 2, Team Fortress 2, Portal and Steam Coming to Mac in April

YESYESYESYES thank you, Valve!