[citation][nom]sdedalus83[/nom]There's no difference between the Mac and Windows steam clients when activating a dual platform Valve product with a retail key. If you have a Mac and want to pre order through a brick & mortar or online retailer, the PC version will work for either.[/citation]
Good point! To say it a little differently, you can purchase the "PC-only versions" from brick and mortar places, and play it on the Mac. You'll have to register the game onto Steam in order to play it, and then it'll work on both Mac + PC.
Only difference is that perhaps you will not be able to install the game on a Mac from the disc with a retail copy. You'll have to download it through Steam. In that sense, yes Valve is the only retailer offering the Mac platform.