If enough large games were released for OS X, even as a port, Apple may put a little more effort in. And yes, the graphics cards are weak (mine is an Intel HD 4000.) But look at the Mac Pro. They invested a ton on engineering it for performance, including a lot of effort on the cooling. Apple can do it, they just need a little push. And now is a great time for it, they have no place else to go. They can't keep marketing the CPU speed or how thin the machines are for much longer. They need a new metric to push and gaming performance would be a great one.
The only way Apple would become a good choice for PC gamers is if they started shipping computers that competed on price with gaming PCs while offering comparable performance.
I could easily see "casual" PC gamers (people who play games casually, not who play casual games) buying a Mac for their general purpose PC and playing games on the side, iff the Mac offered decent gaming performance (they don't) and were priced around the same as a decent gaming PC (they aren't).
It's a chicken-and-egg problem, though. Studios don't want to invest in making (or even porting) games for a non-gaming platform, gamers don't want to buy a computer they can't play games on, and Apple doesn't want to invest in pleasing a consumer segment that doesn't exist.
