Well this is quite something when graffiti has become a work of art and one which is in demand and is legal!
In order to know more listen to the following:
Read the article below:
http://www.redbull.com/cs/Satellite/en_INT/Article/Graffiti-artist-Inkie-interview-See-No-Evil-part-1-021243069927835
Then find out a bit more about the history of graffiti and tagging in Bristol in the four part document below:
http://www.youtube.com/watch?v=ijIUO9VM-X4
http://www.youtube.com/watch?v=h7GdY0rvr3U&feature=relmfu
http://www.youtube.com/watch?v=-l6upiKXCkI&feature=relmfu
http://www.youtube.com/watch?v=5JglVOg14Do&feature=relmfu
Enjoy!