Normally I don't like graffiti, but check this, on a Glaswegian cyclepath. http://img20.imageshack.us/img20/8158/IM000148.JPG And a picture of me, next to it (hope you can see me OK!) http://img20.imageshack.us/img20/5449/IM000152.JPG
they forgot to write wall i wouldnt really call that graffitti either, graffittie is more of an art than just writing stuff on buildings.
they did write wall if u look but some one graffed over it. Actually graffiti comes in many sorts, to graffiti something is to damage property by drawing/writing on it.. and no matter what ur view on graffiti as an art is this is true. Graffiti art it as you say an art full of some wonderful tallent (and some not so wonderful tallent) Then there is allso graffiti which is not art it is indeed just words or tags.
i am actually a graf artist and that is NOT graffiti. i mean, it is, but thats not art. the art is in the peices (short for masterpieces) artists do. a one color word on a wall is a tag. bubble letters in 1-2 colors is called a throw-up. good graffiti is at www.graffiti.org