Google will let you know

Expect an e-mail from Google over the next month. It seems that Google is trying to improve their ad sales. No matter which Google site you use - except Google Wallet, Chrome and Google Books - Google will note that fact and combine it with your other moves over Google products. The combination, Google hopes, will have great appeal to advertisers. Because of this market potential you will not have an opt-out option. Although, if you have infinite patience and time, you can delete the relevant Google cookies over and over again.