A macbook air sounds great, but I am concerned with the screen-size as well. I would definitely need more than 11" as I wouldn't be using an LCD monitor 95% of the time.
It definitely seems like the macbook air is the go-to lightweight development machine these days.
The 13.3" is 1440x900 which gives you a little bit extra screen space. Asus Zenbook and Sony's VAIO ultrabook both look really nice, also in the 13.3" range.
I work primarily from my 13.3" screen in a wide variety of IDEs, you get used to it.
I love mine, but once I get settled I'm going to switch to a desktop. That said, I don't know how well the Haswell MBAs work in Windows; I would assume that it'd be fine, but check with someone who actually knows?
It definitely seems like the macbook air is the go-to lightweight development machine these days.