It's unclear exactly what is causing the iPhone 4 pre-ordering problems today, but it's pretty clear that the who part is all AT&T.
If you try to pre-order an iPhone 4 from Apple's sales portal and choose to upgrade an existing line, AT&T's account verification system hangs for a long time and eventually upchucks a nondescript error stating that "...there was an error processing your request. Please try again later."
Early reports indicated that you could pre-order an iPhone 4 via AT&T's Web site (for delivery only though) but that too appears to be down.
Clicking on "Pre-order now" then "upgrade" causes it to churn for a while eventually returning an error saying they "are experiencing a system error that prevents us from completing your request." Adding that "For non iPhone related upgrades, please call Customer Care at 1-888-867-4384 and provide the error number at the beginning of this message."
When I eventually spoke to a representative at a local Apple retail store about two hours ago they told me that "Everyone's getting kicked out after the 'congratulations' screen. It's a known issue. Try back in an hour." Speaking with another local Apple retail store just now, it's certain that AT&T's servers that are "bogged down" due to "all the traffic" from the pre-orders.
So why does Apple continue to tarnish its reputation by sticking with AT&T? The almightly dollar is the only logical explanation. AT&T will simply pay Apple the most per iPhone subscriber and Apple is willing to take more money at the expense of customer satisfaction.
If you've got another explanation as to why Apple sticks with the noose that is AT&T, let us know in the TalkBack.
Update 1:20pm ET: I was able to successfully complete an iPhone 4 pre-order via the new Apple Store iPhone app (free, iTunes).
Update 2: iPhones pre-ordered for pick-up are available starting at 7am.