I agree with blackgravity's opening statements - if it's not yours, and you don't have permission, it's not right. In fact, it's a criminal act. Maybe I'm okay with having someone spray paint the side of my house or business, but don't assume it's okay.
With that being said, I've seen some fantastic graffiti as well. In fact, one of them ("Just A Couple of Days", outside Athens, Ohio) actually made the regional papers, then later a book was written about it (
link). It sparked long discussions, several editorials, and even a small batch of websites devoted to figuring out who did it. None were ever successful, and more than 10 years later, I still wonder, "Just A Couple of Days...until what?"
Sometimes, stuff like that can be beautiful - there's a building on 5th Av in Columbus that someone graffiti'd over someone else's work - painting a wonderful picture depicting a nameless family from the neighborhood... nobody has messed with that in at least the 8 years that I've known about it.
But it still doesn't make it right.
blackgravity is also right about there being professional graffiti artists. A while back, Extreme Home Makeover came to town and was doing a house. They actually went out into the neighborhoods to find out who had done some of the local graffiti, then hired them to paint inside the new home.