Google AdSense has AdSense Themed Ads at seasonal times. Today is St. Patrick's Day and many AdSense ads are sporting a theme for St. Patrick's Day. The ads looks somewhat like this;
But do you think it will improve your click through rate? That is what folks are talking about in the forums, that and how much they dislike the look of these ads.
Forum discussion at DigitalPoint Forums and WebmasterWorld.