It seems to me that we need to recognise graffiti as a genuine art form, and meet the artists in the middle. Hand over vast areas of ugliness to them to practice their craft - do what you will with godawful overpasses and other concrete monstrosities - so that we can legitimately ask those who excel in their field to illustrate our landscape and share their gift. And in return, I think it reasonable to ask that they don't tag the fences of random punters.
I have a vast expanse of cream wall, which is just begging to be tagged, and it has been. But that's dull. If you want to use my blank canvas for some real expression, I'm open to suggestions. We need to consider all of these kinds of spaces as potential creative sites, and give credit to the artists who deserve it. Maybe we need a "wall exchange" website. :)
Either way, next time you see some graffiti - boring and ugly or stunning and inspiring - think about what we do to foster this artform, and how we might steer the socially unacceptable into a valued and recognised form (which may well require steering our definition of "valuable").