We got one white and one black too!
It's just a pre-order. The phones aren't actually out yet. I think the phones are "available" September 21, but I think that if they're available to stores the preorders will be available as well. Wouldn't make sense otherwise except that a preorder guarantees you a phone.
They definitely did not sell out in the first hour - at least not at my AT&T. I waited until after work to go to the store and put in the preorder because I wanted to keep our current plan (we have the old unlimited data plan)
You're right AT&T did not sell out technically, demand was higher then Apple expected. Pre-order just allows you to buy a phone and not wait in line but instead wait at home. Apple ran out of phones in that they cannot make enough fast enough to ship all preorders and ensure they get to people in a week. This means that those who've preordered after the first hour (as stated in this article and others) will have phones shipped in 2 weeks in many cases. Glad you got lucky, others are stuck waiting.
Like you said, if it's available to stores it should probably go to people who preordered first, I bet though that each AT&T store is guaranteed a certain number of phones to hand out on the 21st and somehow got priority.
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