@@@WARNING!!!! VERY LONG WINDED RESPONSE!!!@@@
Im a free lancer game developer and i just recently bought my first mac (a macbook pro i7 glossy. I find it EXTREAMLY difficult to get high quality games to run on this platform because of the lack of power. currently im working on a FPS i call ColdShot (based on coldwar/ vietnam missions)(including veribles that would simulate closely to real combat rather then just blowing things up) but i find myself having to sacrifice game resolution and quality inorder to have were it is in playable terms (at least 25fps on 13" model). The 320m might be the fastest intergated graphics card but it is still intergrated graphics which bogs it down. also i can't assume that players are going to get the $1800 macbook pro just to play a $40 game... though the imac did move to a decent playing platform at the price of $1200, but people buy more then just the imac line. Right now my team uses the Blender game engine as our main engine, though it makes porting easy from windows to mac the lack of power is still there.
THIS PART MAY ANSWER YOUR QUESTION
the 330m also has problems in being a decent gpu also, being that it is the entry leve gpu from nividia. so far looking on steam most of the games are not that intesting or revolutionary, unless you like games called "Jolly Rover", "Puzzle Agent", and "Tidalis" most being just ordinary puzzle games that you might see on a flash game sites. there are a few good source titles like counter strike and team fortress but sadly just thoughs few and about 3 more. One of the main reasons gaming companies don't make mac versions is because of the lack of customers and profit they would gain from it, also they are very restricted in how graphical intesive they could make it do to not that many people own a top of the line mac. so mac gaming still is foggy in the future but has made a small step with the introduction of steam.

we are also working on a mmorpg that should be revolutionary most of the details would left unsaid because of fear of big companies such as EA and Ubisoft might take em but it is called "project STRAYS". one of my friends who is a MAJOR mac fan says why not make a mac platform (since we are just mostly a bunch of college kids with little experince in networks.) but there is one problem we would need mac or xsevers that would cost $3000+ for just one blade and using any other sever would break apples eula and force us to buy from them that would be over $20,000usa inorder to fit over 5,000 players comfortbly with a small vent, and support, and storage for the maps and ncps. also we cant just port it and hope for the best, we would need at least 2 high powered mac pros to test and fix bugs on which would be another $8,000usa or we would be developing blindly. so the idea of porting the mmo to the mac platform was dropped before we could even start cause of lack of funding from the school, the club, and the blender foundation (since their main focus is the Durian project).
So our game "Levitate"(basically it is a sandbox game with a little ship) which is one of our test projects is ready to go on the mac and so is the alpha for a rpg "(no name for it yet) but called the code name (Milan)" is also undergoing testing. the name of our group is DARKWIND PRODUCTIONS and we are a brand new group, we plan to make more games for the mac in the future and help you guys out. our site will be up soon (i hope...) so when it is up you can check it out and download some mac games.
