I had a lot of doubts right before switching from a big dual-screen desktop setup to a 13" mbp for all my development work. So far, I've had zero issues with it. Basically, it never feels like my productiveness is hindered by the screen size.

Given that the 13" air has a better resolution than the mbp, I wouldn't consider it a problem. Of course, it's a matter of opinion and taste. But at least one skeptic was proved wrong very quickly. My next dev machine will probably be a 13" air or equivalent, if my trusty mbp dies.