If your current machine has a magnetic hard drive, even the bottom-end Air will feel incredibly fast by merit of its SSD. Check out how quickly it'll boot:

http://www.youtube.com/watch?v=iOEi5Kxpt0A#t=3m57s

As has been pointed out, compile speed isn't necessarily CPU-bound and some compilation tasks are quicker on a slow machine with a faster drive. CPU performance is much less important than most people think.

Screen size is a more difficult issue, as so much depends on your development approach. I'm increasingly inclined to think that my large display may actually hinder my productivity, as it seems to facilitate distraction and procrastination. I seem to feel less bad about procrastinating if I have my text editor open. I'm giving very serious thought to replacing my 17" MBP with an 11" Air and a Kindle DX. A lot of writers use a full-screen text editor like WriteRoom, or even a typewriter, so there's a lot to be said for minimalist, low-distraction tools.