Some discussion of deep packet inspection which, due to the relentless increase in processing power, is becomming much more common than ever before. Like it or not, you really don’t have much choice in the matter, do you?
http://blogs.zdnet.com/BTL/?p=9786
[edit: link died]
Unlike what the article says, I don’t see much of this based on my ISP. However, sites where I have a profile (MySpace, facebook, etc.) do a half-ass job of picking keywords out of my life and presenting “interesting” products (mostly not).
Gmail shows me links to Spam (the Hormel type) recipes when I visit my spam folder. duh.
If you want to see this done really badly, check how Yahoo! News links together photos in slideshows. Underwear recovered from the plane crash in the Hudson? Half the slideshow is about some lingerie show in Timbuktu last month.