I was just flipping through an old issue of De:Bug (I’m 10 months behind in my reading) and there was a fascinating ad for Adidas.  Apparently it was a campaign that ran in Berlin in March, 2006.

First they put up a blank sheet with a Da Vinci quote (“For those colours which you wish to be beautiful, always first prepare a pure white ground.”):

Then they would wait a few weeks for graffiti artists to ad tags, etc.:

Then they would come back and put up an outline of their shoe over the graffiti:

End result, a creative, pseudo-free ad.  Check out a full range of photos courtesy of Being Hunted.