For casual gaming, I can see Apple make a dent (albeit small) in portable gaming with iPods, but then again, the hardware is pretty limited (to my understanding) - it doesn't have the dual-screen and stylus input of the DS nor the awesome screen and raw power of the PSP, so the breadth and depth of games it can come out with should be limited compared to the DS and PSP offerings. But the userbase is definitely there.
Plus, there is no reason why iPods have to be perpetually confined to the form factor and functionality it current has. Apple can simply start releasing portable music players with slightly more "game-capable" hardware shoehorned into their next batch of iPods. They already have one element in place - the multi-touch screen of the iPhone, which has great potential as a gaming input mechanism.
But if this is indeed a battle for the PC, then I say Apple has an uphill battle to fight:
http://news.softpedia.com/newsImage/Mac-OS-X-and-Linux-Bite-the-Dust-Windows-Vista-Did-It-3.png
Here Apple has their work cut out for them. First, you have consoles, then you have the 800lb gorilla that is Windows to contend with, all vying for the same piece of pie. Second, you have the already installed user base that has their gaming rigs and assorted hardware invested in Windows, and with the high price of gaming PCs, I don't think anyone will go out and plonk another 2-4k on a Mac just so they can get arguably the exact gaming experience. Third, the Mac userbase is not associated with games to start with (or they'd be in the Windows camp!) and at least initially, the only people playing Mac games (if gaming ever materializes beyond the three-year-old ports it occasionally receives) will be the existing users.
Additionally, Microsoft is trying to create some synergy between Vista and Xbox, both dependent on DirectX, and developers are used to working with that API. Mac has OpenGL which is nice and platform-independent, but the majority of gaming software and hardware are designed around DirectX first and foremost.
Nothing is impossible, however, and it could be a matter of a generation or two (in terms of PC, not human lifespan) to change the public perception of Mac to that of a viable gaming platform, but I'm not holding my breath on this.
Anyway, my 2 cents...
