I spent two hours trying to pre-order my iPhone 4S last night. According to AT&T's press release, I wasn't the only person swearing at its site every time it told me it couldn't process my order.
AT&T announced that they processed 200,000 iPhone 4S pre-orders in 12 hours. That's approximately 278 iPhone pre-orders a minute. That's only AT&T. Verizon and Sprint have yet to post their numbers.
So now you know why a process that should have taken five minutes, ended up taking all night.