I phoned FedEx, talked to lovely, pleasant lady who told me the following:
FedEx parks large trailers in the lots of the major Electronics Companies (Apple, HP, Dell, AT&T) with which to deliver major orders. These trailers are filled up by the respective companies. During loading, the company creates FedEx entries, then loads them on the truck. So in this case, AT&T scans the iPhone, creates a FedEx tracking number, and stacks it on the truck. whenever the truck is filled, it is driven to the FedEx sort facility. This is not scheduled, and happens when the truck is full.
When the truck gets to the FedEx facility, it is unloaded and all items are scanned. This is when the package shows up in the FedEx tracking system.
This means that REALLY lucky folks like me, who have a shipping date of the 18th from Apple but no entry found for FedEx, have an iPhone on the truck loaded by the LAZY crew at AT&T (just kidding, I' sure they're all diligent hard workers) and it is still sitting on the pavement at AT&T in Texas. (or it got in a car accident on the way to the sort facility)
Hope this helps. She also said that the idea about the system missing boxes isn't true.