It's happening more and more frequently. Videos are being uploaded on youtube, and then getting a grassroots buzz through the social graph, and then they are being picked up by the mainstream media (whatever that means these days).
My wife pointed out this example: a couple upload their wedding dance video onto youtube, which was based on the main dance scene in Dirty Dancing, the film, gets 2,000,000 views, and then ends up on Oprah.
A few stories like this have gone mainstream, and made it through. These pieces first circulate amongst friends, family, and colleagues, and then explode into the mainstream. This last stage is still a bit of a mystery to marketers. We try and seed viral campaigns, but seeing them 'go viral' is still a leap into the dark. Harnessing it will be the key to success for brands. What we've started to do, at my agency, is ensure our PR strategy is locked down as well as our seeding strategy when we launch a viral.
Interestingly, there are hundreds of firms who load youtube with advertiser material but very little of it ever gets more than a few thousand views. The challenge is on to discover what it takes to get crossover.