I haven't posted anything by him and am rather sick of hearing about him but I came across this from 2005 I reckon it's the coolest thing he has ever done. He did this in Palestine. His website says the West Bank barrier is "the ultimate activity holiday destination for graffiti writers".
Banksy also records on his website how an old Palestinian man said his painting made the wall look beautiful. Banksy thanked him, only to be told: 'We don't want it to be beautiful, we hate this wall. Go home.'