Is Google reading my email?:

Comments are down at the moment so I'm posting this to the blog
Many, many, websites, including the google search engine, already do what you're saying. That is how ad servers target ads to viewers - they base the ad they serve on what you're looking at. Go to amazon.com and you'll see an extreme version of this in action. If you read the GMail privacy policy, you'll see that they aren't doing anything new in concept, they're just doing it with email.

Gmail Privacy Policy