Well, I think it's great the computer based gaming scene is being expanded. One of the several gripes that keeps me in the PC camp is the dearth of games on that platform and everything that comes with it (MODS,patches, etc, etc). With Steam, at least Valve games are now almost everywhere. I'm still concerned on the proper gaming though. You can download it, but can you really play it? A co-worker recently bought a Mac and we wanted to have a LAN party, so we chose some classic games, among them Unreal Tournament 2004. The game runs fine on his laptop, but it got insanely hot suprisingly fast. It happened to another co-worker who also has a mac. This is a game that's 6 years old and should no longer be considered graphically intensive. Frame rate is fine, but I don't know if the Mac architecture is designed contemplating that kind of demand on the system . Those aluminium casings with no ventilation whatsover apparently underperform in these conditions (from my experience). I don't know if Apple would consider changing around the parts inside as to contend with the heat that playing most recent games will undoubtedly generate, since it might end not looking pretty enough to be a Mac.
Also, if Mac gaming does take off, peripherals would eventually come too. Since I'm pretty sure existing gamepads, mice, headsets, etc, will not play nice with mac (or basically the other way around), propietary peripherals would have to be designed as well. If Apple makes them, I think they create an even more expensive gaming experience that the one usually found with PC gaming, and perhaps even more elitist that way.