I'm not sure if this has already been posted, but Jakob Nielsen has posted an excellent article regarding Web usability and "banner blindness." http://www.useit.com/alertbox/banner-blindness.html There's definitely some useful insight in there that can be applied to AdSense.
Yeah I remember seeing that some time ago now. It's always good to bring this information up again, so older publishers can get a refresher, and newer publishers can learn! What I always found a little weird was with the middle image. Does no one ever look at the image? I for one always look at images, even just for the hell of it. Makes you wonder a bit.