Hate to have such a rant as the first post (Direct TV sucks! Sorry couldn't help my self) so here is a cool Wednesday chaser. Reverse Graffiti! The basic idea is that the artist uses soap and a brush to selectivly clean the grime caused by pollution, advertising, or what ever and write your message using that negative space. Its legality is in question, but obviously it is far more legal than using paint. Over time, (as shown in this photo) more grime covers up your work. And the only way to "clean up" the wall is to wash the dirt off! So Dirty = Clean. Clean = Dirty.
"Banksy" is one of the big names in this in the UK, and here is his web page.
Also, Here is an NPR slide show of some of his work.