Among the more interesting details:
-- Requires Mac OS X 10.5 or higher
-- 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
Some of these details are not at all surprising, particularly the minimum requirements of having a 10.5 install and an Intel CPU. Those still using PowerPC Macs may cry foul, but I imagine the work it would have taken to get Steam running properly on PowerPC would have been both difficult and expensive, with little payoff for Valve. It's also not surprising that the integrated Intel GPUs won't see official support -- a beta tester noted that he was able to get Portal running on his MacBook with integrated Intel graphics, but the frame rate was only 10-15 FPS on the lowest settings.
Counter-Strike, "The Half-Life Series," Left 4 Dead, Left 4 Dead 2, Portal, Portal 2, and Team Fortress 2 have all been confirmed for release on OS X, with presumably more games on the way later on.