I'm also extremely happy that it will be released on OSX too. I hope this is part of a new trend now that Macs are selling better than a few years ago (and the Intel hardware can't hurt).
"With its release, Valve will include native Mac OS X, OpenGL versions of Left 4 Dead, Left 4 Dead 2, Team Fortress 2, Counter-Strike: Source, Portal, and the entire Half-Life series. Portal 2, due by the end of 2010, will be simultaneously released on both the Windows and Mac versions of Steam."
May 12th! Between steam for OS X, SC2, and eventually WoW: Cataclysm, I'm not going to be getting anything done this summer. =(