Guerilla Gardening is a war against the neglect of public space. It’s an informal organization, a network of underground cells of subversive “seed-bombers” who (re)claim orphaned land. Their mission:

Founder Richard Reynolds began by beautifying London, and his blog inspired others around the globe to take part what’s become a movement.

He’s released a book, On Guerrilla Gardening, which has received extensive press coverage.
Sounds like a great idea. I am a big fan of guerrilla composting so even the weeds on my lawn will get some sweet compost lovin’.
Maybe this book written during WWI will help aspiring guerrilla gardeners
http://books.google.com/books?id=DTJJAAAAMAAJ