ax also pointed us to this article on Inside Mac Games, which states that only the PC version of Myth III has gone gold as of yet:

As you probably know, the Mac and PC versions are being made [and] distributed by different companies, so there won't be a hybrid box like with past games in the series. Donges has explained to IMG that the Mac version is very close behind its PC counterpart, though, and should be heading towards GM in the next week or so.

So PC users will be able to play Myth III a short while before Mac users, but there should only be a period of two or three weeks between releases.