Hello,
I would also add that viral marketing is spreading faster than regular marketing ideas. That's due to the fact that people are eager to send the message forward themselves, becaus ethey like it or some other reason, and are not "forced" to do so.
Of course - viral marketing campaigns can also be incented in a way - for example "Forward this to 4 of your firends, and you will be a part of a car winning competition" - which is normally used for lead generation.
It is however a little more difficult to launch a viral campaign with a direct reponse in a sense of purchase and not only lead generation, but it is possible to do it anyway.
P.S. if any of you have some good info or case studies on viral campaigns for startups, I will be more than glad to listen.
Matic