I came across this website called PriceProtectr a while back. Its a pretty handy service that can end up saving you some extra money on your purchases.
They summarize the service:
"There are lots of stores out there that offer price protection policies -- when the price drops on an item you've purchased, they'll refund you the difference. But there's a catch... it's up to you to watch prices. Price Protectr makes it simple to keep track of your purchases and get your money back. It's free. It's easy. It's free and easy money. How often do you get that offer? "
So it works like this, you buy something with one of the supported stores, then you tell Price Protectr what you bought and give them your email. If the price changes and the price guarantee for the store would entitle you to a refund you then get a notice from Price Protectr about the price change so you can then contact the store for a refund.
The site is free and simple to use. I already used it myself to get a $20 rebate on a HD DVD player I bought last year on Amazon. Right now I"m also using it to watch the price of HD DVD movies that I'm interested in buying and hoping the prices will drop as they liquidate them.
Price Protectr is a great site and did I mention it is free? :)