Some of you would have seen the latest post on the Adsense blog about what to do and what not to do.
On the do side is advice that the top paying ads on the page will go to the ad unit nearest the top of the HTML.
At the page in my signature, that is sooo not true. The 120x600 in the left column is at the highest in the html but it's the 336x280 that pays the most - far more than the 120x600.
Can you tell from your channels which unit pays the most? Where on your page is it, near the top or elsewhere?