If I could tag this for Doc Searls in del.icio.us, I would, but not being able to dig up his account info this’ll have to do:

We need to distinguish between “viral” marketing and “guerilla” marketing. The reality is that no agency can create viral marketing, this is the sole domain of the consumer. Viral marketing is what happens when a campaign works - when we allow their message to travel via our own superefficient conduits.  (from: Penny Arcade! - The Inevitable Next Step)