MacNewsWorld has a pretty good in-depth overview up about Mac gaming: where it's at and where it's headed. The basic story is that the three things that have historically held Mac gaming back behind PCs (the technology differences, the OS, and the smaller installed base) are slowly disappearing. With Apple's switch from PowerPC to Intel, the introduction of Boot Camp and increasingly easy development in OS X, and the growing popularity of the platform, gaming is actually bigger on the Mac than it's ever been.
But there is still a huge obstacle, and that is DirectX. Though there are ways around it (Freeverse actually mentions the Unity engine in the article), many developers are stuck developing in DirectX, and that leaves the Mac platform out of the loop. And there really isn't anything comparable to it in OS X, either. Graphics hardware remains a problem, but that just harkens back to the biggest problem of getting games on the Mac: support from Apple. Some developers say that there aren't games on the Mac because Steve doesn't want them there, and until Apple shows evidence to the contrary, PC will always be the gamers' platform of choice.