If you've looked anything up on Wikipedia, you've seen those little  tags. Any user can slap a  tag onto a "fact" or assertion they take issue with. This is intended to encourage the author or another contributor to put up or shut up - prove the assertion is true by citing a primary source, or take the assertion down.
This is one of the great methods used to keep Wikipedia entries reliable and (more or less) factual.
Matt from biphenyl.org has printed a bunch of "citation needed" stickers and distributed them to his friends. You'll start to see these stickers pasted to advertisements, graffiti, anywhere you see a claim of dubious veracity.
This is all part of Operation Mindf**k, which sounds intriguing. Sort of strange in a "these people have too much time on their hands" way, but intriguing.