I'm doing some research for a client who is considering dabbling with user-generated content (that's all I can tell you or I'd have to kill you). Thought I'd post some stuff here.
Mozilla has a site called Firefox Flicks that's only been promoted via word-of-mouth. They've created a home for fans of the browser to submit a 30 second spot or a video podcast. They don't seem to say how many submissions they've had, but one blog post I found said they had 280 submissions as of April 28, 2006.
Also, check out this post on iPlot about "Crowdsourcing." Lots of good references, including our old client, Marketocracy.