This happened to me just before Christmas. If you preorder for the quoted price and the price comes down, you get the difference back.
From Amazon
Pre-Order Price Guarantee
Amazon.co.uk's price for not-yet-released items sometimes changes between the time the item is listed for sale and the time it's released and dispatched. Whenever you pre-order an item eligible for our Pre-order Price Guarantee, the price you pay will be the lowest price offered by Amazon.co.uk between the time you place your order up to and including the release date. A Pre-order Price Guarantee message will appear on your order summary during the checkout process. If you use 1-Click ordering, Pre-order Price Guarantee protection will be automatically applied. The order summary in Your Account will reflect the lowest price within 24 hours of any price change. Terms and Conditions apply.