I have to disagree here. Other than being made out of Aluminum and being light weight for their size there is no comparison. The Air is a real computer with full OS-X operating system. The iPad is, take your pick, a-- toy, lifestyle device, iOS device, neutered tablet, giant iPod Touch, web/application appliance...
If these Air's were released prior to the iPad, I think it would have hurt iPad sales. With the instant on, full keyboard, FULL WEB (install Flash), etc, an 11" Air can lay around the house just as easily and do so much more. The 11" Air Screen Resolution is higher than the 13" Mac Book and MB-Pro's as well @ 1366x768 vs. 1280x800. Think how much better watching a movie on an airplane will be with the Air! No fumbling around trying to "prop-up" an iPad-- just open the clam-shell, and experience 16x9 video.
The 13" Air, actually offers up some real computing ability. While no MB-PRO, I did play with iMovie 11 on it, and it would be just fine for editing a family movie, etc. Also, the screen resolution on the 13" air is higher than the 13" Pro, and the same as the 15" Pro @ 1440x900.
Just my .02,