Sony has done an extremely good job in their interactive promotional work for The Da Vinci Code. Here they have demonstrated what power there is in viral integration into the context of the story by involving the audience as participants in the movie before it has even launched.
With polished production and sweet use of interactivity, the audience is hooked not only into the movie’s trailor, but part of it’s contextual concept; solving puzzles. Better yet, they partnered with Google Mail to embed a puzzle or code that the audience needs to solve each day. The answers lie in the trailor which you need to review periodically.