Some games like WoW or EVE have taken this into consideration, but other games like Rift and TOR have not and do not offer a Mac client. Why would a company, serving a world of gamers alienate their potential customers by not offering a game on their platform. This same issue has bugged me in the past when a title is offered for Playstation but not for xBox. It just seems silly to cut your customer base up like that and by adding additional hurtles for the minority players to overcome. It seems like bad business sense.
I digress, I have been gaming on my Macbook Pro for almost 3 years now. I enjoy it, and for the most part the Mac has been able to keep up just fine. I had a little trouble with EVE, but only when I was in the hanger with the "realistic" rendition of my character. The spaceships looked fine. WoW is the same way, the computer keeps up fine unless I turn up the graphics to a ridiculous degree. I think the issue is my video card, but I don't' think it would be easy (or cheap) to get a new video card for my Macbook Pro. I honestly don't mind turning down the graphics a little. The game still looks great to me. You have to give credit to Blizzard that their choice in art-styles has stood the test of time. Most games that are 7 years old look terrible by today's standards. I'm gonna stop this one here since I think I'm rambling. Cheers!