I have found that one of the tricks of being a successful art teacher is to find projects that are relevant to students and if they think that they are getting away with something a little devious in the process all the better.
At the start of this unit I would take the time to have a dialogue with my students as to the difference between unartistic "taggers" and artists who practice graffiti art. Our school, like many in California, was made up of several buildings that connected via outdoor hallways, so between the open campus and being in the middle of a couple strong gangs it made our school walls a constant target for gang tags (symbols, names, initials, etc.). I always emphasized the importance of being respectful of others, including buildings, and ways that artists who are interested in graffiti can make a living being hired to do just that.
I came across an article in the Arts & Design section of the New York Times on two Brazilian artists, who go by the name of Os Gemeos, who recently installed a 17 x 51 ft mural on the corner of the Bowery and Houston in NYC. It's beautifully rendered and filled with vibrantly colored whimsical scenes. The mural will remain up until March 2010 and showcases a wonderful way in which graffiti art can be tasteful, masterfully crafted, and provide a means of earning a living as an artist.