This “heat map” illustrates ideal ad placements on a sample page layout. Google folk have published an elaborate Optimization Tips page for Google Adsense publishers.
Looking at my recent adsense statistics, I must admit that the above illustration is fairly nice. 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.
Google has done an impressive presentation to woo new Adsense Publishers.
Adsense for Content remains my primary source of revenue from this blog. I do not use "Adsense for Search" as Googlebot doesn't index my site too well. Yahoo! spiders do a better job, atleast in my case.