I was directing a fellow mac user to buy the new copy of Bone (because Bone is good, but mostly to vote wih dollars towards future porting of other excellent games *cough*samnmax*cough*).
Anyway. There's no indication at all that what you're buying on the site is access only to the PC version. In fact, the only reference that you can even buy a mac version of the game seems to be in a blog post. What's up with that?
Did you guys sign some kind of deal so that only that other publisher can distribute that Bone game? Seems kind of weird since it's so different from your other deal with a third party publisher i.e. Gametap, where you can still distribute the game on your own site.
In any case, at best, you'd have the site look at the user agent string of the browser and figure out that when someone is buying the game on a mac to automagically let them buy the mac version of it. At worst, you'd have a prominent link to the other publisher, proclaiming them as the mac publisher of Telltale games.
That being said, I feel almost as a second-class citizen not being able to use the main Telltale infrastructure for long term support of my purchase, such as redownloading the game or getting access to my activation key. The other publisher's infrastructure is at best spartan, and at worst nonexistant. Obviously there is an issue of client support, but you could just forward mac support requests to them.
If you're trying to use this first port as a measure of whether you want to develop more Mac-compatable games, you're really shooting yourself in the foot with all this preceding stuff.
By the way, Bone was a lot of fun. Something funky happened when I installed the Mac version, and it crashed a lot when trying to activate it (which a reboot fixed), and it wiped half of my other applications' settings after installation, and it crashed during the hide & seek minigame (autosaves ftw)... but the game itself was a lot of fun.