I think Apple has inventories on hold at FedEx warehouses ready for release, same day. Orders start at 1:00a.m. PST, shipments go out same day for those that order quickly enough. Arrival happens on Saturday or Monday depending on each individuals location.
Since BB, Wal-Mart, and others have the iPads, I don't buy the ship from China theory. I expect early orders will ship from the US.
The reason Apple cannot commit to "overnight" shipping is that consumers may not get their devices on time...then they'll complain about Apple. I imagine we will see 2 day shipments standard with inventories pre-positioned for release.
Just my $.02 guess