My experience:
I'm a university sysadmin. About two weeks ago, one of my faculty members (Dr. "C") got a new 13" MBAir, i7, 256GB, 8GB RAM. He has not been able to get it to stay connected to either of our University wireless networks (one uses WEP, the other WPA2) nor his WEP home network.
The machine connects to the wireless network for 5-10 minutes, then loses the connection (though it still seems to display a connection) and cannot reconnect until it is rebooted.
Yesterday, I received my new MBAir, identical to his except for a 128GB hard drive. I unboxed it on campus yesterday, and could not join either the WEP or the WPA2 network. Unlike Dr. C., I didn't get an intermittent connection. I got no connection at all. The WEP network would claim to work, but didn't; the WPA2 network would fail at authentication, telling me my password was wrong.
Last night, I brought the MBAir home. On my home network (which uses an Airport Extreme that's only a couple of months old) the connection was flawless.
So today I took mine back to campus, and it was still unable to connect. Somewhere (perhaps on the MacRumors forums?) I'd read that some people reported success using a "virgin" user account, one that hadn't been migrated over from a previous computer. The claim was that some network settings could get migrated over that were incompatible with the new hardware.
Both mine and Dr. C's new machines had been migrated over from 2009 MacBook Pros, so this was at least something to try. I performed the following steps on my machine:
-- Removed / "forgot" all wifi networks.
-- Deleted the network preference files.
-- Created a new, blank user account.
-- Logged out of my main account and into the new account.
-- Attempted to connect to the WPA2 wifi network -- it worked!
-- Logged out of the new account, and back into my main account.
-- The machine still worked on the wifi!
-- I left it streaming some Internet audio most of the day, and the connection was rock solid. No problems.
I then went downstairs to Dr. C's office and tried the same steps. His machine, however, did not respond positively. Even under the new, virgin account, his connection would drop after < 10 minutes.
Dr. C will take his laptop to the Genius Bar tomorrow, in the hopes that they'll be able to provide some assistance.
My best guess at this point is that there are potentially both software AND hardware issues affecting these machines. My issue seemed to be software-related, and was (hopefully) quickly resolved. Dr. C's seems to be hardware.
Just wanted to throw a bit more evidence on the fire!