Shaw, aka DigitalPoint, has posed an interesting observation. He has seen a correlation between the natural rankings of a page for a keyword term and the earned revenue per click on the AdSense ads for that page. He said, and its just an observation, that he has seen the revenue per click go up as the rankings for that page rises. The theory is that Google's smart pricing system takes into account the natural rankings for a page for a specific keyword phrase. The higher the rankings, the higher the price should be per click on the AdSense ads.
Here is the specific example given:
One site ranked around #25 in the SERPs for it's keywords. AdSense paid roughly $0.10 per click. As it slowly moved up in natural rankings, so did the AdSense per click payout, until now it's top 5 and it gets $1.50 per click on average (nothing else has changed).
Conversely, I have a site that did rank top 5 for something, and it was getting about $1.20 for AdSense. The rankings have dropped, and the AdSense per click payout mirrored that drop (now it's averages $0.02 per click).
I wanted to point people to the thread over at DigitalPoint Forums to drive some more thoughts on this topic.