Recently Shawn at Digital Point has seen some Google AdSense ads which have Google logos in the background. Generally many people associate Google with good (or being credible.) These images would probably increase click through rates on sites where the ads are not well blended into the content. On sites where the ads are well blended into the content then the clickthrough rate would probably decrease substantially.

He posted what some of the ads look like over at the SE roundtable weblog.