I was toying between that and an MBP 15", and went for the MBP in the end.
Main reasons where
- Extra oomph of the quad core i7, extra memory and Radeon 1GB GPU
- More ports and internal optical drive
- Larger storage
- Larger screen
- Thought it would handle better under load
- Bang for your buck
My friend, however (an Oracle DBA) went for the 13" Air and says it's great. Incredibly snappy and responsive (think he says it boots in 12 seconds, and has a great battery life). He's got VM Fusion installed, has Linux (CentOS or Mint, I don't recall) and will install Oracle in it. I wait to see his impressions of how "snappy" it is when all that's running!
The Air is expensive. The base model comes with only 2 GB, and the optical drive is extra. But it is really light - great for travelling, and the display is better than the 13" MBP.
It's a gorgeous little thing, and had I not needed the extra "oomph" I absolutely would have gone for one of them.