Graffiti most certainly can be art, and one of its most well-known proponents is the "guerrilla artist", Banksy. He spray paints his quirky sense of humor (as seen to the left, on the wall in the Wet Bank) or dark perspective of humanity onto anything that may or may not be nailed down. His art can be briefly seen in the background of Children of Men and it's worth checking his stuff out on his website. I just wonder how he managed to maintain his anonymity when coming in to do his thing on the set of the film...?
I'd much rather see his graffiti than the shite I see that's not even legible- the big bubbly lettering that looks vaguely girly but you know it was done by some 16 year old who thinks he's as hard as nails. You're not cool and you're not making a name for yourself when no one can bloody read what you've written!