The other day I was walking down Mission Street looking for street art when this big, bright bus shelter ad caught my eye. And not just because that bigger-than-life Egg McMuffin looks positively delicious, but because of the seemingly extraneous McDonald's logo dead center.A closer look revealed that this was a decidedly unsanctioned use of the Golden Arches.So what does this say about the aforementioned dialogue? Here we have a street artist of some kind turning a corporate entity's own adver