I posted this in the Tmobile thread but here's how the dates are handed out by the carriers:
Apple notifies each carrier that they will have X amount guaranteed. Obviously anything greater than that amount goes into a que and fullfilled in batches, not one for one. The initial upfront amounts were devices already at shipping hubs. With non stop flights from China all weekend, expect to see those 9/19-9/26 and later to move up to release day. You'll see movement Mon, Tues and Wednesday. This happens every year. The ship dates are always on the conservative side to offset anyone not getting their devices when promised.
I got this information as I know someone in supply chain management for VZW and currently works reverse logistics for them.