Well, that was fast.  AT&T has sold out of its iPhone 4 pre-order allocation, Boy Genius Report says, and any order placed after 1:30 PM Pacific (4:30 PM Eastern) will be delivered between June 25th and July 5th.  Engadget reached out to AT&T for a comment on the issue and AT&T had this to say:

"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."

After multiple reports that both AT&T and Apple's systems were having issues with the pre-order process, it's kind of surprising that all of the devices are gone already.  As you can see in the photo above, Apple's site is still saying that pre-orders will arrive on June 24th as of this writing, so it might be worth contacting Apple if you haven't gotten your pre-order in yet.  If that doesn't yield any results, you could also try third party retailers like Best Buy, Radio Shack, or Wal-Mart to get your paws on an iPhone 4 on launch day.  For those that tried to order today, were you successful?

Via BGR, Engadget