I was at my local apple store yesterday getting my wife's phone replaced. Employee said they still had a ton of phones. Also went to a radio shack to get my dad a phone. They had a lot of Verizon phones but no AT&T and sprint.
It depends on the time of day. For instance, yesterday Grand Central had no AT&T phones when they opened at 10:00, but got some right around 2:00 when I tried again. I got my phone in less than 10 minutes. If you are close to a store, your best bet is just to check in every once in a while as they may get a shipment in at any time. Yesterday the associate told those of us in line that our timing was perfect as they were turning people away a few minutes prior and just received the shipment unexpectedly.