Six months ago I had a iMac 24" and a MacBook Pro 13" for studying and portability. I noticed after I bought my MBP that I didn't use my iMac as I thought to be using. Not because it was slow or bad or something. No it was just horrible to synchronize both devices with my work and study related stuff (Yeah I know now better that it is a charm with Dropbox).
While I was deciding if I should sell my iMac I decided to sell both iMac AND my MBP. Why, you ask? My MBP was just fine but sometimes I had to carry a heavy books plus MBP with me and this was why my back was hurting. So I decided to switch to something lighter and faster (and yes my MBA is way faster than my MBP ever was).
After selling both devices I bought a MBA with 128GB SSD and 4GB Ram and happier than ever before. This was one of the best decisions I made in my environment. And if anything happens to my MBA I will order a new one with thinking a moment about it.
Pro's:
* It is very fast,
* it is light as nothing else,
* I have instant-on (something which is really important to me),
* I have 5-6 hours of battery life,
* Eclipse runs really really smooth,
* XCode runs even smother,
* and every other development related works just fine (and sometimes I think even better as on my MBP)
Con's:
* No glass display (I loved it cause it was way easier to clean than a "plastic" display)
* No keyboard backlight (while this was bothering me at the very beginning, I have to admit that I don't care anymore)
At the bottom-line I want to say that unless you are working on really high computation stuff (like 3D rendering or something) a MBA will do it smoothly. I never encountered anything which I had performance problems with. (If you ask me I would wait a second longer in exchange to carry much less weight with me)
Disclaimer: I don't play any games and try to escape Flash everywhere possible (cause it lets the fan freak out).