Less than 24 hours after iPhone 4 became available, it is completely sold out. Both AT&T and the Apple Online Store are out of them. You can still pre-order, but only for the second batch, which will ship by July 2.
The fact that Apple is through their first run of iPhones after about 20 hours is impressive alone, but it wouldn’t be far-fetched to say that they would’ve been gone twice as fast if the pre-order system was actually working.
When the iPhone 4 first went up for pre-order, there were massive failures that prevented people from completing their orders. Those failures continued into the morning and throughout the day. Some people managed to get it to work on AT&T’s website, but most couldn’t.
You can still pre-order, and the system should be fine now. It’ll be for the second shipment, though. Although July 2 is just 8 days after the release date, that’s when it ships, so it could be a few more days after that. If you really want an iPhone before July, your best bet would be to hope that Apple releases the white iPhone before then.