The official Google AdSense blog reports on the ad optimization of website Engineeringtalk. Quote (my emphasis): “They found the biggest influence on CTR [click-through-rate] was the similarity between the AdSense type (size and font), and the main body text of the page. The more similar the type, the higher the CTR.”
I’m certain this tip, and many other AdSense optimization tips, can be wrapped up by saying “make the ads look more like your normal content.” However, some of these approaches – certainly not using a similar font, but e.g. putting the ads right inside the content – can turn into barriers for reading the main content. And of course, that hurts the site (and ads) in the end.

blog.outer-court.com/archive/2005-11-03.html#n86