The National Portrait Gallery in Washington DC is currently exhibiting a show called "Recognize: Hip Hop and Contemporary Portraiture." There's a nifty segment about graffiti. Yes, at the Portrait Gallery! Their thesis is that graffiti is an art form that is performed without an audience, so the artist's "tag" and distinct lettering style becomes (in a sense) his portrait.
There are four huge panels created by CON and AREK, and the cool thing is that each panel is exhibited on the Gallery's website along with a slideshow that depicts the creation of each piece. You get to see the guys block out the lettering and develop each piece layer by layer. I'm still amazed by how sharp-edged the lines are. There must be some secret they're not showing...? Like, maybe there are giant wads of blue painters' tape just off camera?? Still, it's cool to get a glimpse into this normally audience-less artform.