This is fascinating! I can see the power of this technique in advertising. Can you imagine a manufacturer doing this with a popular product, and asking their product users to post replies?
TechPresident contributor Steve Garfield writes about it here.

John Edwards sent out an email yesterday promoting an “emergency” ad protesting President Bush’s veto of the Iraq war bill. The ad, featuring an array of Americans saying “it’s time to end the war,” was uploaded to YouTube (of course), with a twist: Edwards asked his supporters to upload videos of themselves saying, “we the people” in response to the ad. TechPresident’s Steve Garfield wrote that ” This makes it very easy to participate and collaborate with the campaign to send a message to Washington. Anyone with a webcam can post a video response. People are already discussing the ad’s merits, in the comments, on YouTube.” Check out all of the responses; it’s a refreshingly new way to engage people with political advertising. I wonder if this has anything to do with Joe Trippi coming on board…