This is a tricky topic to answer. From my experience many of the key caps are interchangeable on the pro/desktop keyboards but not always as there have been a couple of revisions. I have never found the MacBook Air's keys to be interchangeable with other models however as they have a completely different sissor mechanism and connection points (at least with my 2010 model though that has completely different keys anyways since it lacks a backlight). If you need replacement keys I would stick with MacBook pro ones. Is there any specific reason you want to use air keys?
Here is a good article explaining the different sissor spring mechanism revisions, although not the air ones.
http://www.powerbookmedic.com/Macbook-Un...