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After being told by 2 AT&T CSRs via chat that The USPS would be bringing my iPhone, I called them. A fantastic CSR, whose husband is also awaiting a 4s, told me that mine was scheduled for overnight shipping so FedEx would be the carrier. However, many people would be receiving standard shipping through the USPS. She said that the system assigns shipping methods and that they have no control over or understanding of it. She also implied that USPS customers should start seeing tracking info tomorrow, but I wouldn't see anything until Thursday.
From my understanding, ATT uses only FEDEX and UPS for shipping. I have never heard of people getting their iPhones via USPS. For one thing it would be a logistical nightmare. Consider all the people at apartments or condos who have shared mail boxes with one of those larger size lock mailboxes for packages. The USPS could only get a fraction of the iPhones delivered placing them in there. Another thing, the Post office is not designed to deliver to every single house going to the door and getting signatures for packages. If the USPS were to try and handle 1 million iPhone deliveries on top of all the regular mail and packages, lets just say most people wouldn't be getting their iphones on friday.
Leave package delivery to the companies that solely deliver packages and leave the mail to the mailman.