If the price rises tomorrow, I believe you'll have to pay the new price unless you have a proof like a printout to show the lower price.
Look at the FAQ I printed above from BestBuy.com. It says if the price goes up, you pay the price you pre-ordered at. That seems plain as day to me. They sent me a confirmation email and I printed a receipt.
Search for "pre-order" at Best Buy.com and you'll find that document. It also says that your credit card won't be charged until they ship the item, which is nice.