Here's the demo of id tech 5, id Software's new graphical engine, running on a Mac at the WWDC keynote. It looks very nice (it should, at 20gb of textures), and Carmack says the engine will allow programmers to come in and get the game right first, and then let their artists loose on it. Levels can be designed before anything else, and then artists can come and design the landscape and the colors around that.
I'm not sure how well that works, but we'll see soon: Carmack also says he'll have this showing on both consoles and desktops (Mac and PC) at E3, as well as "another Mac related announcement" that he can't quite bring himself to tell us about at this point. Considering all of id's games are already available on OS X (released by Aspyr, who have to be shaking in their boots after all the announcements this week), we have no idea at all what that might be.