Viral marketing can be very powerful and effective.  However, if your message spreads like crazy but you never actually benefit from it you've missed the point.

Seth Godin's blog entry, What is viral marketing, makes this abundantly clear.  He should know.  He was one of the first to experience the power of this marketing method with a book he wrote and offered for free.  Eight years and millions of downloads later it's still available for free.  He admits now that he didn't fully understand the benefits he'd receive from his book.  He didn't write it with the intention of manipulating the public into buying something to make him rich.

The critical element of viral marketing is built in.  Examples include Hot Mail and You Tube.  The more people use it the more it spreads.  I suggest you read Seth's blog on this, because as he states, viral marketing is getting a bad name.  Mainly from those who are trying to do it for the wrong reasons.  It's like telling a band to write a hit record using a specific formula for the express reason of selling lots of singles.  That's soul-less.  You can be smart in marketing, but it's not about being deliberately exploitive.