John Battelle has an interesting piece about Google Ad Planner on his blog - http://battellemedia.com/archives/004574.php#comments
"The strong bias toward Google network sites is suspicious."
The article is about inconsistencies between the Ad Planner data and Comscore data for the same sites, claiming that Google is potentially favouring sites on their own Ad Network.
"In short: If you were a media planner using Google Ad Planner, and you were looking for larger sites, you would be led to sites that are running Google AdSense, on average, over sites that do not. Net net: This data indicates that Google Ad Planner pushes ad dollars to Google sites over non-Google sites."
I have signed up to the Ad Planner beta in AU and plan to give it a solid test to see how it performs and whether it has potential for agencies. Right now I am suspicious ... a publisher (which Google is more of than not) running audience measurement doesn't sit too well ... but at the same time the Nielsen and Morgan tools are so bad I'm open to alternatives.