I recall walking through a street of San Francisco (though I can't remember which one - maybe the Haight somewhere) when I saw a big spray-painted piece of art on a wall above the top of a building. I instantly recognized it as being in the style of Banksy. I doubted it at the time because I'm a London expat and it seemed too much of a coincidence to have your "local" artist suddenly turn up in your new neighborhood.…
Perhaps Columbus and Broadway: http://www.flickr.com/photos/lilvia/5140456220/