I don't know why this never sank in. I think it went in one brain cell and out the other. Last year, around November, Google changed the clicking behavior of their text ads for Adsense. It used to be that you could click on the title, link, or the description portion and it would click through. Now you can only click on the title portion.
The net effect of this, according to the pundits, is that clickthrough rates for text ads went down up to 60%. Clickthrough rates go down, revenue goes down.
However, image ads remain 100% clickable. The solution? Change your setup to serve only image ads. Now that the quality of google's image ads has generally improved, it should not be an issue.
If you are using their new Custom Channels, you can actually do this without having to change any of your ad code that's in your pages.
As a general rule, it's a good idea to watch your CTR and eCPM figures carefully after making such a change - it doesn't work the same for every implementation.