The whole point of the preorder is to try and get your iPhone on launch day. If AT&T has "x" phones of a particular type and your order is "x+1", then the delivery will not be on launch day. It gets very confusing with the combination of five colors, two sizes and three memory capacities. That's 30 different combinations! So, if you ordered a combination that was less popular, then it is more likely that it would be delivered on launch day. If you ordered a combination that was more popular, then your delivery may be later because the supply ran out.