TUAW: Steam for Mac details.

• Requires Mac OS X 10.5 or higher / Intel-only.
• Source games require 10.5.8 or 10.6.3 or higher, and are not supported on the X3100 or 900-series Intel chipsets (the integrated graphics chipsets in older MacBooks).
• You will not be required to purchase separate versions of Windows and Mac games. One purchase will work on multiple platforms.
• Steam and Source will run natively in OS X.
• No emulation whatsoever.
• Source will run on OpenGL.
• Mac and Windows users will play on the same servers and join the same lobbies.
• Release expected in May.

The big deals in bold.

Excellent.