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!
My computer specs: 1.7GHz Intel Core i5, 4GB RAM, Intel HD Graphics 3000 384MB
My preferred Diablo III settings: Fullscreen, 1440x900, 150 FPS (foreground), 30 FPS (background), Low texture quality, Low shadow quality, Low physics, Low clutter density, Anti-Aliasing on. (The game defaulted to higher settings, which I also tested in the video; it introduced minor lag in some enemies and support characters)