It was bound to happen sooner or later. The Federal Trade Commission has issued a statement (or "staff opinion") that there should be disclosures on viral or word-of-mouth advertising and marketing. This means, essentially, that if you plan to use viral videos as a form of marketing, you might have to blow your cover - which seems to negate the whole idea of creating a covert campaign in the first place.
We have several clients interested in using web video to generate interest; the couple of ideas we've actually developed aren't disingenuous - it would be pretty clear that they were tied to an organization or event. But in some cases, I can see this being a bit of a buzzkill to the creative process. I've always thought it was an interesting form of promotion, and it goes back much further than people realize. I was totally taken in by OK Soda back in the day, and while I didn't exactly fall for the hype, I thoroughly enjoyed the buzz around The Blair Witch Project.
I'm interested to see how this plays out.