Yet another prankster has been posting quirky stickers on ads in the subway. Roberta Thorn, whom I believe is the artist behind this work, has countless photos of stickers in the same style as this one on her Flickr. The majority of these seem to include bits of commentary on the art world, but some also have a more political flavor.
It seems we have covered her handiwork before, unknowingly. Her repertoire includes subway ad alterations, fun graffiti and a whole lot more. She definitely seems to be one to watch.