I don't see why it wouldn't be. It was almost playable under my Parallels XP virtual machine under Medium settings, but I gave up because there was some serious stuttering if I had the map open. I believe that it's playable under the latest version of CrossOver Games, too.
I had RIFT running on another Windows laptop that had a Nvidia 310, which is a slower card than the one in the Air, and it was completely playable once you dialled down the graphics settings.
The GPU in the Air is one of the better cards available in integrated form, so it's really just the CPU that would be the bottleneck.