Ektopia
Developed by the Graffiti Research Lab a division of the Eyebeam R&D OpenLab, LED Throwies are an inexpensive way to add color to any ferromagnetic surface in your neighborhood. A Throwie consists of a lithium battery, a 10mm diffused LED and a rare-earth magnet taped together. Throw it up high and in quantity to impress your friends and city officials.

LINK to video.
Of course somebody has to clean that all up… How long before graffiti means pasting electronic paper on a wall, which will then show a movie until the battery runs out…