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.