So here is what I believe is happening and why we are not seeing significant numbers of order status changes (and some folks wont until very late in the game). This is all speculation on my part, but I have been in the distribution business for about 15 years.
In the China facility, they are likely shipping using 2 different methods:
Direct fulfillment, these are the orders placed through Apple. Individual phones are placed in cartons, manifested to the parcel carrier, palletized, loaded and sent on their way direct to the buyer.
Bulk Fulfillment, They ship the phones in bulk to a third-party distribution center (AT&T) where they are broken down, cartonized, manifested to the appropriate (cheapest) carrier, and loaded on a trailer for shipping.
The phones are back ordered because there is literally no stock available in the AT&T warehouse to fulfill your order... yet. There is pending inventory that is/will be en route to their network, but they cannot allocate this inventory until it is physically verified at their facility. That is why we see a backorder quantity.
As for a tracking number, the key to the processes that I outlined above is the manifesting process. The folks who ordered through Apple already have one because their cartons were manifested in China, direct to them. AT&T orders will not see a tracking number until they are manifested from their facility, so probably the day before they arrive. You could see it sooner if they do a ship and hold type of process, but I dont know enough about what they do. The hold could be at their gates, or they might task the parcel carrier with 'hold for delivery on 9/21'.
Anyway, my belief is that if they are already manifesting direct shipments, the bulk stock for the carriers is already en route. I'm assuming that they already have parcel trailers on-site that are ready to load. They have a tremendous volume to get through, so they will start very shortly after the stock arrives and is received into their facility.
The cartonization and manifesting processes that they are doing at AT&T will likely be going full speed for the next few weeks. So tracking numbers will probably start trickling in over the next 24-48 hours. If you dont have one by late Thursday night, you wont be getting your phone on Friday.
Editing to add: When they mainfest, that is when THEY know the tracking number. It doesn't necessarily mean that they are publishing it to US at that point.