Have a look at this Google “heat map” that I took from www.google.com/adsense/tips. It shows, according to Google, the locations on your website that users tend to focus on. Google uses this as a guide to show you where you should position Adsense on your site — or any other kind of advert for that matter… or if you don’t like adverts just put important content there. It is an interesting demonstration of where users tend to look on a site.
Some of Google’s “revelations” are obvious, but there were a few surprises for me: for example that the top right corner of the page is a rarely looked at area. That should make the advertisers in those positions ecstatic.
Google writes on the site: “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.”