I play Diablo 3 and WoW occasionally on my 2010 Air (1.6Ghz 11", 4Gb RAM). Both run very well on low settings. I have set a FPS limit in WoW at 30 fps, and it usually keeps around there. The limit also keeps my CPU quite cool, the fans are barely noticeable and I'm at about 75 degrees Celsius while gaming. About the same in Diablo.
This is best for casual gaming though, as lots of enemies on-screen can cause lag. The most stressful fight yet (WoW, 25 man raid with tons and tons of spell effects and such) I had 8-10 fps when things were at its worst.
I have ordered a 2012 Air (11" with everything maxed but SSD) so I'll be very interested to see how/if the performance of these games differ.