According to this growing Reddit thread, Apple’s iPhone Upgrade Plan is struggling to keep up with demand. Customers are being denied, and it seems that Apple Retail employees’ hands are tied, waiting on Citizen’s system to recover. (Citizen is the company Apple is using to finance the iPhone purchases.)
Reading this reminded me of the giant cluster that was the iPhone 3G launch. Clearly, this issue with Citizen is nowhere as bad, but I’m sure its still frustrating for everyone involved.