Not sure if anyone from the Walmart thread posted or not, but I just received my 5$ preorder card in the mail and on the back it has instructions on how to redeem the exclusive DLC. This includes a code to enter into xbox marketplace. Also it stipulates how to pay for the rest of the preorder.
Soooo, if anyone was doubting or wondering about getting charged the rest of the game price without going into the store, or whether the DLC code would be given to customers AFTER picking up the game, the answer is negative on both questions.
Preorder card INCLUDES the code that will unlock VUDU credit as well as DLC, and the instructions lead me to believe charges won't be made unless you present the card to Wal-mart.