According to Boy Genius Report, all of AT&T's pre-order iPhones have been reserved as of 4:30pm EST. And if you placed an order any time after that, you might have a bit of a wait on your hands.

AT&T's official statement confirming the sellout:

"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."

source: BGR