I placed my DF order on launch day when ATT ran out of 16G.
The next morning I got a backorder confirmation email, so I called ATT. They said I would have to go to the store that I placed to order at to cancel.
So I went to the ATT store and was told that you can not cancel a DF, you just have to refuse the order when it comes in and they will credit you back minus shipping and a restocking fee.
I lucked out and while I was at ATT to cancel I found out that they had some 16g in stock so I just bought one then. That was Saturday the 12th. Today my DF order arrived at ATT I went over, told them I didn't want it, and was credited back the $300 minus a $10-15 restocking fee (I'm not exactly sure of the fee since I also had to pay shipping)
Here is something to keep in mind. I was a NEW ATT customer switching over, This allowed me to place the DF order on Friday without signing a contract. Had I been a pre-existing ATT customer, then I would not have been able to go ahead and get the phone the next day at the subsidized price since I would have already had an Iphone attached to my account.
Hope this helps.