Owners of MacBook Airs, if you have the 13-inch that I have, you're fine. Diablo III will run on it and be perfectly playable. Take a look at the video here to see it run on my late-2011 Air at both default settings and low settings.
I ran the beta on my Air last September, which has led to many people e-mailing me, asking if the finished game can run. Here's your answer!