Check out this sidewalk artist named Julian Beever - anyone ever seen any of his work? My youngest son is majoring in graphic arts and brought him to my attention.
http://users.skynet.be/J.Beever/pave.htm
One of my favorites, where you can walk to a part of the sidewalk and appear to be trapped on top of a building... incredible!!
http://users.skynet.be/J.Beever/batman.html
There are also some videos of his work on youtube.