http://clickz.com/showPage.html?page=3628004

http://mashable.com/2008/01/03/isps-behavioral-advertising/

ISPs buy hardware and software from a variety of vendors that helps them closely track customer activities. From there, the ISP buys up remnant (i.e. – bottom of the barrel) advertising from the large third-party ad networks like ValueClick, 24/7, Burst Media, etc. Using the inventory they’ve acquired, ISPs can then serve their own behaviorally targeted advertising to their customers.