To answer this question, one must first answer the question, "What is art?"
What does art do?
It inspires, provokes thought, plays with one's emotions and feelings. It could brighten someone's day, it could encourage someone to take over the world, or it could cause someone to yell out "what the #*$&@*$ was that !**#@$&?"
If graffiti can do that to a person, than it is art to that person. However, it can me it's nothing but vandalism to another.
But thats the beauty of art, its always open for interpretation.