Has anyone tried otherwise?
I caught the Amazon flash sale on the PC version, but I don't have a windows partition terribly handy. I was expecting I'd just be able to go somewhere in Steam and key in the serial # and download the Mac version.
It's worth trying it to see - I bought the PC version and activated it through the Win7 bootcamp partition. I seem to recall it was asking me on steam if I wanted to download the game after putting in the serial number, and it was after then I got it to look at the disc to load it from there instead.
TBH I wasn't paying that much attention and it was a while ago so I may be wrong, but I remember being irritated that it seemed to want me to download the game when I had the disk.