Update: We've gotten the official statement from AT&T, and it's true, though you still might be able to waltz in on launch day without a pre-order and pick up one of the unallocated units:
"Because of the incredible interest in iPhone 4, today was the busiest online sales day in AT&T history. As of Tuesday afternoon, customers who preorder iPhone 4 moving forward will receive their device on June 25 or later, depending on when the order is placed. We'll email customers with confirmation once their order is placed, and again when it ships. In addition, we will have devices available on a first-come, first-serve basis in our stores beginning on June 24."