I don't quite understand all the people selling their iPads for Macbook Airs.
I use my iPad primarily for reading - Flipboard, magazine apps, Reeder etc
These apps are either not available on the Mac, and even if they were, it's just a far better experience reading on the iPad than the Macbook Air ever would be.
Maybe it just depends on how you use your iPad/Macbook Air - but for me, they simply can't replace each other. If you're trying to use the iPad as a true laptop replacement by taking lots of notes on it and trying to do photo editing and file management etc then the iPad was not the right choice in the first place.
Buy whatever it is you need. If you need a portable reading experience with some browsing and games and interesting apps get an iPad. If you need a full computer experience on the go in a portable package, get the Macbook Air. If you need both, get both devices.