Clever. The question is, did the owner do this, or is the box itself meant to be art as well. Oh who cares just check out this picture:

Photo by Greg Long of gama-go.com

The text reads:

All are welcome to express themselves in the box below.

and

Painting within the above box is hereby expressly permitted and shall not be considered “graffiti” in accordance with article #23 of the San Francisco Municipal Code

Via Public Ad Campaign via Boing Boing