This is a transcript from ATT online rep, also I call up ATT corp store and they same thing, you can use debit card. Not sure if the reps are reliable.
Jane: I read somewhere u can't use debit card like visa bank card to preorder the new iphone?
Kimberlee: Yes you would be able to use a debit card to pre-order a phone.
Jane: According to AT&T's press release, customers who want to preorder should use a credit card as debit cards are not allowed to preorder.
Jane: this was from the press release pdf file i think
Kimberlee: I will look, but I doubt that is correct.
Kimberlee: One moment please.
Jane: k
Kimberlee: You will be able to use your debit card to preorder.