I started to build exactly the same a couple of months back but then abandoned the project because a business on a hack is risky!

We had considered the same issue, but decided to give it a try anyway.

Apart the visited link way there are also other means of achieving the same result even working server side. Check out this paper http://wozad.com/docs/sp3.pdf ,it's covering also the privacy issues.

but hang on, before you get all up in arms about privacy and rights, this is a possible google adsense / adwords killer and there is going to blood shed before privacy gets in the way of profits.

Yeah, because no way could Google have figured out how to do this.

Google should really also integrate this hack into google analytics -- that way they can give rough estimates of where else your traffic is surfing.

I believe there's something in Google's S-1 filing that might technically prohibit them from doing this.

Surely they also look at your browsing history, when you use Chrome, or even when you use their search engine.

Surely you haven't even looked at Google's privacy policy, which spells out exactly how they don't do this with Chrome.

btw I like it!