Apple and AT&T both experienced a complete and total meltdown of their servers under the significant load of iPhone 4 preorders. Dire effects were experienced by iPhone 4 customers according to Twitter, including seemingly successful orders failing after completion, AT&T log-ins redirecting to the accounts of other users, and even full-priced, off-contract handsets ending up in shopping carts.
It appears that Apple still managed to sell out of their initial supply of iPhone 4's despite the unavailable online store. The delivery date for orders has been pushed back to July 2nd on the Apple website which 8 days after the launch date of June 24th.
The $99 3GS can still be had on time, if you still want that classic...after seeing the new one.