Update: Steam for Mac is now live. You can download the client directly here. (Link fixed.)
The client download doesn't seem to have materialized just yet, but according to reports, today's the day for Valve Software to introduce an OS X-compatible version of its Steam game download service. Assuming the client file goes live at some point today, Mac owners will finally be able to enjoy the streamlined Valve service that has dominated digital gaming distribution on Windows systems since its debut in 2003.
You can read Valve's press release on the launch on its Web site. In terms of game availability, Valve mentions that Runic Games' Torchlight, the fun, lightweight Diablo-esque dungeon crawler, will be available today. Valve is also bringing out its own Portal, a clever first-person action/puzzle game.
After the initial launch, the release states that Valve will roll out more titles weekly "to highlight specific functionalities of Steam on the Mac." A few Mac enthusiast sites were granted access to the beta program for Steam on the Mac, and reported yesterday on their experience with Valve's online shooter Team Fortress 2. Valve has also promised a Steam for Mac-based version of its popular Left 4 Dead 2 in an interview with AppleInsider. It also intends to launch its upcoming Portal 2, and all subsequent games simultaneously for the Mac and the PC going forward.
Other than Valve's own titles and Runic's Torchlight, we know little about games from other publishers that might appear on Steam. That hasn't stopped Mac gaming enthusiasts from speculating, of course. This thread on Valve's Steam user forum does a fine job correlating the games currently on Steam for Windows with games currently available for the Mac through hard copy or other means. It seems reasonable to imagine that those respective publishers might have made the same connection. For the sake of Mac gamers everywhere, we hope so.