I figure I'll throw out my 2-cents after lurking for a few weeks...
I ordered a 6 16gb gray via ATT Premier on 9/17 and immediately recieved an email confirmation stating the shipping dates would be 9/26-10/7. Then, a week later, I got a 2nd email from ATT stating "this is just a friendly confirmation that we have your order processed and we will ship within the dates we gave you".
After seeing others that order the same item having their shipping dates pass and replaced with a "back-ordered" status on the last shipping date, I had a feeling I would have the same happen to me. Sure enough on 10/7 my status changed to "back-ordered". Luckly I've seen others that had the shipping date end on 10/2 get theirs shipped yesterday or today so there could be a 5-day waiting period once your item is listed as back-ordered on the order status website (not via the order state #).
So here is my theory on the order timeline (at least for iPhone 6):
- You will not get it shipped within the timeframe ATT gives you

- Your status will be changed to "back-ordered" on your last shipping date
- The phone (hopefully?) will be shipped 5 days later
If this is the case, I should have my order shipped on 10/13.