Jenstar reports on a DigitalPoint thread that shows how Google AdSense is experimenting with 3rd party tracking solutions. This test is for Google to see if they should outsource the tracking of impressions and clicks to a impartial 3rd party company. Jenstart has a quote from an AdSense representative stating;
We will be working with a number of different 3rd parties during the course of this experiment, since different advertisers use different third parties to measure effectiveness. Google will not have any access to the 3rd-party cookie information, and these 3rd parties do not collect or track any individually or personally identifiable information. Of course, publishers can opt out of this if they so choose. Finally, this is an experiment, and we will evaluate the results of this experiment in a couple of months to figure out how to proceed.
She also notes that this is only being tested in image ads and these ad formats; eaderboard (728x90), banner (468x60), skyscraper (120x600), medium rectangle (300x250), wide skyscraper (160x600).
Forum discussion at DigitalPoint Forums.