Last summer I tried to cancel my ATT online order and was told that I couldn't. I wanted to pick the phone up in store & didn't know that that was impossible to do when you order online. I went into the store and was able to place another order with no problem (no effect on my eligibility) and was advised, after store employees had spoken to countless other people at whatever support number they use, that the only way to "cancel" is to wait until Fed Ex or whoever delivers the phone and then refuse delivery. Totally inefficient and expensive for ATT, but that's what they said.
Oh, and it will take at least a few months for the charge to be refunded to your credit card, too.