I doubt that. I have a 3G iPad on order - just because... well, just because. But, it will not be a replacement for my Air. I am sitting in a cafe right now having breakfast as I type this on my Air. I carried it here in a small leather messenger bag, oblivious to its 3 lb. heft. I have sent a few emails, PP'd some photos in Aperture, fixed some contacts on mobile me and read the newspaper on it. Oh and I did not need to prop up the screen up with sugar bowl and coffee cup in order to read it... I could live without the iPad but I could not live without the Air.
I would love to see a good MBA update - it is a pity that Apple won't offer one with 3G connectivity built in along with an opt in/ out arrangement with AT&T.