If you want to know more about the history of Obey Giant,Shepard Fairey just came out with a retrospective book of his work, Supply and Demand. He is currently traveling a lot doing book signings and talks. About a month ago I went to a talk he did at the MOCA which was really good and informative. If he is giving a talk within a reasonable radius of you, I highly recommend checking it out.
Also, here is as clip of Fairey discussing with Farrell the aesthetics of his work: