Steam is definatly on Mac
 

Along with the Source engine and all Valve games post Half-Life 2.

Not only that, but if you own the PC version, you now own the Mac version too. With Steam Cloud enabled, you can actually save your progress on one, switch computers and continue on the other format.

I don't have a Mac but it's GREAT news for those that do.

Honestly, that PC-Mac with one purchase is a fantastic move

http://www.destructoid.com/steam-con...l-166230.phtml