This could be a rather complicated question to answer, actually. Pro Tools only "supports" using an external firewire drive (first problem), then you need a USB interface, and most likely a PT user will be using an ilok (second problem because it also requires a USB slot)...oh, and hardware like the MBOX supposedly won't work on a USB hub...so, mix all of those ingredients together and you probably won't find many people who have tried to run it on their Air.
But, you may find a mad scientist who has tried it. I know I've done plenty of field recording on a laptop to the internal drive with no problems, however that was a 7200rpm drive. Given the newly given life of the new Air, I would say it's going to be hard to find somebody who has tried Pro Tools on their Air, I actually thought about trying it, but with a 4200rpm ATA HD it didn't seem like fun to spend all that time installing for a one minute experiment. I'd guess you'd run into buffer errors almost immediately. I just don't see how an old Air would keep up. But now I'm curious...
However, if you want to do some simple audio recording I would recommend good old Cool Edit Pro 2, which is what Adobe bought, slapped their name on and changed it to Adobe Audition, and started charging money for it. It's old, but it's a great program. A lot of radio stations actually still use it (or Audition) for recording audio.
