If you haven't seen this already, it's from Google's AdSense Optimization page.
Certain locations tend to be more successful than others. This “heat map” illustrates these ideal placements on a sample page layout. The colors fade from dark orange (strongest performance) to light yellow (weakest performance). All other things being equal, ad placements above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.
It looks reasonable, but I have to wonder: how does Google know this? Have they measured their own campaigns? Are they re-casting info from the folks at Eyetools? There's nothing in the AdSense insertion that would tell them where the ads show up on a given page. So either they've collected some data or they are making an educated guess.