tim222 - 11:37 pm on May 19, 2010 (gmt 0)
Below the fold on an informational site can be a prime spot if the article is about the benefits of a blue widget and the ad is about a sale on blue widgets. In my experience the CTR and conversion on affiliate ads/links placed at the end of articles in this manner can be fanstastic.
Higher up on the page might be the best place for advertisers in general, but not always.
I've seen similar results, where ads at the bottom right of an informational article will get better CTR than an ad at the bottom right of some other page (for example, a list of names and addresses). But even so, the top ads still get the higher CTR and eCPM.
I can see Google's reasoning for doing this. I've seen websites with ads from several networks on the same page, with ads stacked one on top of another at the bottom of the page. The content of the page itself might be good, but the bottom of the page is a link farm. Most real people probably don't sit there and look at all those ads. Many clicks are probably accidental. Those ads are garbage to an advertiser.