The WSJ asks if Graffiti is art? A quick video from them re:The Street is Art Exhibit @ MOCA.
 

art/ärt/Noun

1. The expression or application of human creative skill and imagination, typically in a visual form such as painting or sculpture, producing works to be appreciated primarily for their beauty or emotional power.2. Works produced by such skill and imagination.

Yes, I think graffiti clearly qualifies. Perhaps their question arose from thinking that vandalism and art are mutually exclusive?
More info on the exhibit for Art in The Streets