Matt posted up this great idea:
One of my favorite quirks about Wikipedia is the little  tags that users can place in an article, indicating that a dubious claim needs a reference. One day an idea struck — what statements are more dubious or outright ridiculous than those in advertisements? In true wiki fashion, the final placement of the stickers is a collaborative effort, now distributed and anonymous. If anyone sees one somewhere, please make a photo! I’ve been tagging my photoset on Flickr with citationneeded and wikiffiti — more should start showing up in the next few weeks. I’m also providing the source Photoshop file (or as pdf) for anyone who wants to print their own batch.
Wikipedia  stickers – Link.