rustybrick - 12:38 pm on May 14, 2010 (gmt 0)
Some details at [adsense.blogspot.com...]
"In order to determine which ads are above and below the fold, we've implemented a statistically-driven model. The model takes into account various user experiences and situations, including different web browsers, monitor sizes, and screen resolutions, and only considers ads above the fold if they are fully on-screen when the browser window loads."
Plus details on how AdWords advertisers can exclude below the fold ads at [adwords.google.com...]