People have long speculated about the never expiring cookie that you get from Google when you search on their site. Privacy advocates have been waving their hands in the air since Google became popular.
Jeremy (Jeremy Zawodny’s blog: Why Google needs Orkut) makes a fantastic point here, a way for Google to connect the dots between a person, and what they are doing on the Internet.
Doubleclick and Real did this years ago. Every time you where served a Doubleclick banner they would check for a cookie, and give you one if it didn’t exist. Real would then provide their profile information when you downloaded the Real Player and bam, instant marketing database with demographic information for millions of people.
Be careful what information you give out to web sites… you don’t know where that information will be used.