Just heard about this last week in the New York Times. It’s where you knit or crochet tags to fit public objects (statues, bike racks, railings), then go out and sew them on in the dead of night. Check out the original article here: http://www.nytimes.com/2011/05/19/fashion/creating-graffiti-with-yarn.html?_r=1&scp=1&sq=yarnbombing&st=cse.
Some yarn bombing blogs: http://yarnbombing.com/, http://www.streetcolor.wordpress.com, http://twilighttaggers.blogspot.com.
Just a note: it is technically illegal, so be careful!
Peace,
Wildwool
