There was a good article in the December WIRED about the campaign Chevy ran last year to promote their new Tahoe SUV. I remember reading all of the bad press they got relating to some of the negative ads that people had submitted, which (of course) then circulated like wildfire on YouTube :-)
You could see how things like this would have most companies running for the door when it came to the idea of user-submitted content. What's interesting about the WIRED piece, though, is the fact that the Tahoe contest itself drew more than 30,000 entries, vast majority of which were positive. It's a good read for anyone considering going in that direction with their brand.