Dave Balter, founder of BzzAgent, points to his Word of Mouth Manifesto in his BzzAgent Investor/Advisor Update.
There’s a reason the subservient chicken didn’t increase Chicken nugget sales, why the Segway (a.k.a "IT", a.k.a. "Ginger") hasn’t changed the world despite drool-worthy P.R., and why Richard Branson descended a New York City skyscraper in a nude suit. And it all comes down to the distinctions among Viral, Buzz, and Word of Mouth marketing.
UPDATES: the BzzAgent blog also points to this slashdot conversation on Open Source Word-of-Mouth Advertising.