I myself have noticed drop in adsence takings per month - and my trafiic has risen considerably, so im feeling google are focusing on product related adds (
Smarter Pricing) - and kinda rewarding clicks based relitive products on page!
I have placed adsence on a number of dance music related page such as
DJ Mixes
DJ Sites
Soulful House Mixes with adsence dropping on related subjects
John Scott mentions some usfull info on the subject of adds being more productive the nearer to the product relating to the adds..
you can read more about this topic here:
Adsence - Smarter and Smarter Pricing