ClickZ: Google Unveils "Smart Pricing" for Content Ads
Reuters: Google Revises Ad Pricing for Content Sites

Solving the problem of different sites providing vastly different quality of clicks, Google's developed a sophisticated algorithm to match marketers to surfers. Content ads AdSense are distributed across a wide network of sites and to address quality concerns, Google has implemented "smart pricing," a method by which marketers are charged less for ad clickthroughs from some sites where quality is low. Of course some publishers are going to be those low quality publishers, and their revenues will fall. Now that clicks will potentially be different on AdSense traffic, some marketers that had considered shutting down that segment of the network can give it another try.