Viral marketing

There's no denying the sheer power of viral marketing when implemented well, but when is viral marketing too much.  'The Dark Knight' viral campaign ran for almost an entire year before the release of the film, recruiting millions of people in an obsessive hunt for clues, only to be short changed by events not entirely in their control.  Is this adding value to the finished product or simply a waste of time?

Certainly it brought fans of the film together, but would the film have generated the same high amount of revenue through simply word of mouth and the quality of the finished film?  These are questions not easily answered.  I know that I did not actively involve myself with the viral campaign (it probably reached me on a subliminal level from other people talking about it) and I still enjoyed the movie.  Is this extreme viral marketing now a necessity for the success of a product?