"There's this new iPhone 4 coming, and we're going to start getting pre-orders on June 15th, and lots of people are going to want one, so be sure to have everything working on the website in time, okay?" Apparently, that message never got to AT&T's website team, because it's been all but impossible to upgrade existing iPhone accounts to the iPhone 4 all day. It can't be done through AT&T's site, Apple's site, or the new Apple Store app that the company just released for the iPhone. It's gotten so bad that the title for Apple's own ordering page is "iPhone 4 Goes on Pre-Sale, Purportedly."
Could THIS finally be the straw that breaks the camel's back, the event that causes Apple to say "We need a second carrier in the U.S."? Probably not. I don't know what it would take for Apple to say "Enough's enough," but I wouldn't want to be part of AT&T's website team right now.
Update: I just managed to place an upgrade order through AT&T. It's a miracle!