Think about this: So Amazon can learn about people's browsing habits and can infer a user's interests. Now, based on this, it can put ads in the screensaver (say with "$20 off on the Kindle fire if you let us do this"). An extreme case would be to modify the webpage with their own ads (hopefully will not happen and is unethical). But nevertheless good move, Amazon.
I think their IPs will start getting blocked if they substitute ads. However, the Kindle ads would work wonders. Just inferring the main categories in which the user makes purchases from their Amazon history and showing the top five best sellers in those categories would be sufficient, but having the web history is huge. That said, the potential abuses are also massive if they start getting hit by warrants/national security letters.
