Google Gets Into Behavioral Targeting, Launches “Interest-Based Advertising” Beta at Search Engine Land is a must read on this story. In short, Google is testing behavioral ads on the content network, using what they call "interest based categories" and "previous interactions." The cool part is that Google allows you to specify which categories (600 of them) interest you and you can also opt out at http://www.google.com/ads/preferences/.
In any event, the Search Engine Land article is a really good article on this topic. I know I wrote most of it, but both Greg Sterling and Danny Sullivan added tremendous perspective to the article. In fact, Greg touched on a major concern AdSense publishers are discussing about this program.
The concern is that Google's contextual relevancy may dwindle down, in exchange for behavioral relevancy. Contextual relevancy is what Google takes pride in, they love to say they have ads that match the exact content of the page. If you are reading about Sony video cameras, Google will show you AdSense ads for Sony video cameras. But possibly now, if you are a sports enthusiast (Google knows this because you visit sports sites) and you are reading an article about video cameras (because you want to buy one for the next game), you may see sports ads and not video camera ads.
This is incredibly new for Google. They have been using previous queries to trigger ads in search. But right now, Google said this is not tied to search history. There have been rumors that Google used behavioral targeting in search results in the past. But Google said they don't plan on behavioral target a while back.
Publishers who really care about the ads that are shown on their site and want them to be relevant to their site will be frustrated/angry when site users call up to complain that there are inappropriate/irrelevant ads on it. This will especially be an issue for people that share a computer (example: families).
So if you look at baseball they will show baseball ads. So they follow you around and put ads up that has your interest. So oneday later on the day you look at something else and they still are showing those dam baseball ads. Give me a break. Have I mentioned that google is not god and google and adsense sucks.
SO you come to the forum and they still show those dam baseball ads.
There is also a lot of discussion around the privacy and opt out features. The big question is, will publishers and advertisers be able to say, I don't want my ads to be targeted behaviorally?