I would HIGHLY suggest you not do that. Why? Because if you read the fine print (which I have, since I got my Bold through wirefly.com), it states that the phones they sell come with a "$300 Equipment Discount" (for smartphones, $200 for regular phones). And by accepting that price reduction, you agree to repay the discount, and if during the 181 days after the new equipment is activated, you cancel your contract, you authorize them to charge your credit card in repayment of the Equipment Discount.
The above state is correct. Wirelfly will not charge your AT&T bill, they will charge your credit card. So you will be looking at pro rated charges, ETF, plus ETF with wirefly as well. You will end up spending $800 for that phone.