This is one of my favorite Banksy works. It was a block over from my last office. Unfortunately it was defaced by some other graffiti artists. Ironic
http://www.highsnobiety.com/news/wp-content/uploads/2010/05/...
http://www.highsnobiety.com/news/wp-content/uploads/2010/05/...
It's zero-day art. The people who try to preserve it by bolting glass plates over the top are missing the point.