The EFF's Deep Links blog has a post on how some websites charge different prices to different customers. However, the post only look at the negative aspects of such a situation - when a website decides to charge you more because it knows you will pay it. What about websites who offer preferential prices to customers that they know?
An interesting tool to code would be a web application that integrated with Amazon (through Amazon Web Services or something) and showed prices for items. A Mozilla bookmarklet or Greasemonkey script could then be used to integrate the "standard" price into Amazon pages, thereby showing any price differential that might exist.