Seth Godin, 21st-century-marketing genius, explains what viral marketing really is (and really isn't) in his latest blog post.
My favorite quote from the article:
"Viral marketing only works well when you plan for it, when you build it in, when you organize your offering to be spreadable, interesting and to work better for everyone involved when it spreads. If I don't benefit from spreading it, why should I spread it? I won't." - Seth Godin
On a side note you can download a free PDF copy of his book on the subject of viral marketing here: http://sethgodin.typepad.com/seths_blog/files/2000Ideavirus.pdf
So next time you're about to post that stupid-funny video on YouTube, hoping to reel in the bucks for your company, remember the words of Seth Godin: "Something being viral is not, in an of itself, viral marketing."