My opinion:
Netbook = intel atom processor, traditional hdd, upgradable ram and no optical drive, card readers, cost effective
Good for email/web and video viewing, documents
Bad for anything to do with content creating, not as thin, boring trackpad
iPad = multi touch, one piece (it's a slab), single full screen apps, 'even a two year old and his lolcat can use it'.
Good for cumbersome free operations, content viewing and 'fun', exclusivity with the Appstore
Bad for anything to do with content creating and flash based websites
MBA 11" = netbook sized with some traditional laptop components such as SSD, keyboard size, nice screen, trackpad and up to 4GB ram.
Good for laptop based content creation on the go, may work out to be lighter than some netbooks without compromise to the build (plastic vs unibody aluminium construction? Guess who wins on durability and appearance?)
Bad for anything the MBP would otherwise be capable of, and is storage limited due to SSD
This is definately a MacBook killer, but that being said the MB hasn't been updated for quite some time. If anything, the iPhone 4 is the iPad killer.