I asked about this at my AT&T store in Cary, NC and they said that I could not cancel the order until the phone arrives in their store, then I can cancel and they will refund me the money. I'm trying to avoid doing this b/c I don't want to spend $300 on one at the Apple store then be charged another $300 when it ships from AT&T.
You can cancel DFs - walk in and "change" your mind and once they cancel the current DF, show them your new iPhone and say thanks and walk out.