I can chime in here..
I decided to make an appointment and head over to see my local genius.
Gave him the scoop and he indicated there was a few things he could try, and agreed that since it was working great in the store, that is wasn't likely hardware related. So he looked over a few things and explained about a file that kept info on wifi connections and by deleting it, the mba would create a new file instead of refering to an old, possibly corrupt file.. I think it is in : Macintosh HD > Library > Preferences...file called SystemConfiguration ... I'm 95% sure that's it, but not 100%, so please research before deleting.... he went on to say that over the years there hes' found that deleting that file solves many if not most issues... and if that wasn't it, to come back and they'd 'try more', all the while keeping a log of issues so there was a track record.... he seemed to know what he was talking about and I didn't want to spend all day there....While there, I asked about the airport extreme and a bunch of related network questions with hooking one of those into the network. He told me I shouldn't need to buy that to get the MBA working right, but that it would solve the problem and had other benefits (discrete 5Mgz network for one, plus remote printer etc etc etc..)
So I bought one... came home.. no luck on the "fix" he tried... same old thing. Hung up w/in 60 seconds.
So, I unboxed, installed and set up a new network w/ 5Ghz option and i've been online ever since.. yes, it costs $200 and no, I don't think it should be required. It's absurd that they don't have a better handle on this issue... but I kind of like the idea of having a seperate network that I use and it does seem to be a stronger signal through the house....
I don't think I've solved anyone elses issue, so sorry for that. But you could try deleting that file, see if it works and a least when you eventually get to see/talk to apple, you can say you tried that. And, you can decide if you're willing to drop another $200 to solve the issue for good...there are surely cheaper options there too, but I was there and I'm a sucker for white roundish slick looking well boxed eletctronics with a promise of an easier IT life...
I'll keep trying on the 'other network' and update if I figure anything out and keep looking here for ideas.