No problem. I believe mine is a late 2010 model. It has the 1.4 GHz Core 2 Duo with 4 GB of RAM. It's 11'' rather than 13''. Performance is generally excellent and it's probably the best computer I've ever owned in terms of outright utility simply because it's approximately the size of a Kindle DX and I can take it anywhere without a thought. The fact that it's usable and responsive essentially instantly after opening the lid is also huge.
The only real problem comes with pushing pixels around, as the other reply mentioned. I can watch QT video fine, but if I go to Vimeo or YouTube, I can't really get a good playback out of Flash. Generally Flash is bad on this machine. I'm a long time Mac user so I'm not too unused to this, but it seems a little worse than on an iMac or something like that.
I think if you use an IDE for dev work you should check it out at an Apple Store or something to make sure the resolution works for you. That's really the only thing I'd consider. My 11'' is really just too small to me, having been spoiled by dual 30''s on my desktop. If you use a text editor or vim/emacs it'll probably be fine, but IntelliJ or Eclipse or whatever just have too many windows to manage in the space, in my opinion.
My choice is between MBP 13" and Air 13", and looking at the online store, Air actually has the better resolution (~1440x900 vs. ~1280x800). I was surprised at this, but I have used Xcode on 1440x900 on a 15" MBP and I was ok with it.
I have the 4GB 11" Air, and it's quickly becoming my favorite dev machine, even when I'm next to a monster dual quad core Xeon box. The Intel card is fairly slow at pushing pixels around (so if you develop graphics you'll notice slow texture access, etc.), but is mostly capable.
The simple deciding factor is a SSD in the laptop. It's hard to overstate how much of a performance difference these make in compiling, localhost-served webpages, etc.
