Valve announced back in March that Steam would be coming to Mac in April. In somewhat typical fashion, Valve is going to miss this release window, but the good news is they have set the release in stone for May 12th.
The initial run of games will include: Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal and the Half-Life series. Valve has stated that they are treating the Mac as a “tier-1 platform,” and word coming from those in the public beta confirms that the games run natively in OpenGL without the use of translation software.
As Valve has stated previously, all games will be cross-compatible in multiplayer and will not require repurchase if previously purchased over Steam for Windows.
Most of the Steam catalog will not be available on the Mac side, but with Valve leading the charge, hopefully other developers will have incentive to convert their games to Mac as well.