The Vancouver Sun has this article on yarn graffiti. If you haven't heard of this already, an example of knitted graffiti would be a tree cozy or a pole cozy. This is part of an ongoing movement to reclassify craft as something hip, young, and activist.
You can also check out this website for more information. This is a very interesting blog for those who want their work to be more public. And the examples come from all over the world, reminding us that there is a wide and wonderful knitting community and it is global.
(Kudos to Ephram the zimbalist for the link.)