Heh, I had this idea too
It appears overseas they got the idea to hack the overly apparent Microsoft XP avertising billboards. On one of them they used the simple "Spray Can" hack and on the other they were more scomplex hack with the cut and paste technique. Check them out here.
http://www.theregister.co.uk/media/677.jpg
http://freespace.virgin.net/andrew.harrison4/