I did a test over the past week on a site that doesn't earn very much. I got rid of every ad but one small text ad -- the 234x60 Half Banner. In the week, my income shot way up. WAY up. This leads me to believe that the way around ad blindness is to be blind yourself to over-advertising. I'm going to slow try this once at a time with all my sites and see if it makes a difference.
Yesterday, I have done the same in one of my sites, leaving only one link unit at the top and a vertical banner at the bottom. The results are very disappointing at the moment. I will have to wait a little bit to test more.