Google adsense ads have always been pretty drab. They're basically a small rectangle with some text and a link to the advertiser's Web site. Here's Google's attempt to make adsense a little more interesting by applying a little holiday style.
I can't say if the holiday spirit background has made them more prominent to generate more clicks, but anything besides the old boring format is a welcome change. At least it makes them look a little livelier, especially on sites like mine that look pretty boring to begin with.
If you run adsense on your site, you can enable this style by going to "My Account", "Ad Type Preference", and selecting "themed ad units".