If you didn’t encounter the server issue on the Apple website while trying to pre-order the iPhone 4, you’d better hope that you won’t encounter another issue that some are having with it; cancellation. Some people that have placed a successful order saw their status changed to cancelled. There doesn’t seem to be a clear explanation of the cancellations, but it might be that the user didn’t accept the terms and conditions for the exclusive U.S. carrier, AT&T, or that they were violating their agreement with the carrier (i.e. past due balance).
Source: Mac Rumors