One possible problem I see with expanding Mac gaming is that, traditionally, Mac users tend to expect a certain level of quality and performance from their computers. It seems the general message in Apple's products and related services, from the hardware to the software to iTunes and the iPod, is "Don't worry about the details, it will work if you buy it from us." So you don't worry much about specs and things. You just know that you've got a Mac, and it should run your programs smoothly and intuitively. This seems to work with most applications, such as music, video, simple photo touch-up work, word processing, chatting, net browsing, etc.
But not games. I bet some Mac users are going to be expecting the games to run smoothly like everything else, and they'll be sorely disappointed when they realize that their system can't be outfitted with the hardware to run games smoothly at max detail and 1280x720, much less at native resolution on a 30" Cinema display.
Maybe it won't be a huge problem, but I can imagine many a Mac user scratching their head in confusion when Call of Duty 2 or Sims 2 can either run in an 800x600 window or non-native resolution fullscreen with crap performance. Throwing money at the problem won't make it go away. With the exception of the few enthusiast gamers out there, Apple has no incentive to put dual-GPU setups in the MacBook, MacBook Pro, Mac mini, or iMac (and those are all tiny computers... do they even have SPACE for the additional hardware?). Apple gamers will be limited to one small graphics card, none of these elaborate dual and quad setups that are becoming increasingly necessary to run modern games.
Telling users that they've got much more power than new PCs and 30" Cinema displays to boot and then offering crippled graphics card solutions might make a would-be Mac gamer wonder why the platform is suited for games at all. Simple answer is, it's not.