An underground collective of artists called illegal art used hundreds of Post-its to create a giant to-do list in New York City's SoHo. What's even better, passer-bys can "participate" in the art project by writing their own notes!
http://www.rm116.com/2006/09/todo.html
This, and a neat list of more Post-it artworks can be found at WebUrbanist - Thanks Jon Jason!