The importance of Viral Marketing differs based on the product or service being introduced, but for the most part it can be very instrumental in launching or introducing new products to the market.
Viral marketing describes a strategy that encourages individuals to pass on a marketing message to others, creating the potential for exponential growth in the message's exposure and influence. The idea behind Viral Marketing is that one satisfied individual will pass on the message to two or three of their friends, who will in turn pass on the message to others.
It is a snowballing process, that will see your message multiplying until people become aware of what you are offering. Ideally viral marketing is very effective, but is more of a supportive tool to other forms of marketing, especially when you take into consideration that you are launching a new product.
Advertising would do well in getting your target audience aware of the new product, and sales promotions [discounts etc] would get customers to try or to sample the new product or service offering. After this is done, Viral Marketing would come in and fill the gap, giving you what you would call "free publicity."
I think Viral Marketing could work in the case of a teaser campaign, where you would hype the product up, leaving an element of suspense in the process. This is the only way I could think of viral marketing being effective in relations to launching a new product. [People LOVE suspense]