Media event set for September 9, launch likely ~10 days later.
Steam for Mac Details: Native Performance, Chipset Limitations, Closed Beta Underway
Reports confirm previous claims that Steam and the Source gaming engine will run natively on Mac OS X using OpenGL instead of relying on a wrapper such as Cider to translate the Windows versions to Mac. That commitment and designation of Mac OS X as a "Tier-1 Platform" offer evidence that Valve is seeking to make the Mac gaming experience as equal to the Windows experience as possible.
Steam itself will also be Intel-only and require Mac OS X Leopard, although individual games may require newer versions of Mac OS X. Reports also indicate that systems using Intel's X3100 or 900-series integrated graphics will not be supported for Source-based games, although testers have been able to get the Mac version of Portal running at 10-15 frames per second on lowest settings on these systems.
Other details coming out of the beta testing are confirmation regarding Steam Play, which will allow users to purchase a game only once and play it on multiple systems, as well as corroboration that Windows and Mac users will play on the same servers with the ability to join the same lobbies.
Beta users are reportedly still limited to Portal for testing, although word is that Counter-Strike: Source and Half-Life 2 will be added shortly. Other confirmed games for Mac include the remainder of the Half-Life series, Left 4 Dead, Left 4 Dead 2, Team Fortress 2, and the upcoming Portal 2.
Despite a refresh of the Steam client user interface released today, Valve has yet to announce a specific release date for the Mac version of Steam, continuing to hint only "coming soon".