i got a tip message a few weeks ago from google basically saying i should use the large ads such as the leaderboards or skyscrapers ONLY to get the most revenue possible, highest cpm w/e... They said this will allow more advertisers to be displayed on each page so bidding for ad spots would be more competitive. this all seems like common sense but actually i didn't really think of it like that tell i read the message. I have ad units that are smaller (for layout reasons) just like almost every site that uses adsence. i am thinking about taking the advice and seeing how it works even though my cpm is already pretty good and consistent around $1.20 - the message from google also said i could see revenue increases of up to 20% - 50% i forget what the exact number was in the message but regardless that would be a huge increase