I gotta chirp in with the others: does it need to be a Mac?
Until recently I didn't realize how ridiculously overpriced their hardware is, when one of my Boss's was considering getting one for his home, and asked me to look over the specs. I was just floored for just how much of an outrageous price they were charging for such mediocre specs. It's worse than designer jeans!
If you are getting it for the "cool" factor or bragging rights, or anything along those lines: Don't, you're wasting your money
If you are getting it for some specific program: look and see if there is a capable Windows/Linux alternative/port of it and save money
If you are getting it because "Macs don't get viruses": You're wrong. Macs DO get viruses, they just don't get Windows viruses. Save your money and do a Win/Lin dual-boot.
If you are getting it because you are a big fan of Mac OS X: Is it really worth those insanely inflated prices?
If you are getting it for work: Get the most expensive one you can, and get your boss to sign off on it as a legitimate work expense so it's a nice tax write-off.
If you want a thin, portable 13" laptop you can use for web surfing, reports and light tasks, go with the MacBookAir.
I wouldn't recommend Macbook Airs to anyone: Mac OS X does not support TRIM, and Apple hasn't released any information on when TRIM support will be added. Getting an SSD Mac in the mean time ain't something I would do as the drive's life won't last nearly as long as it otherwise would.