Brandweek has a great article detailing the five phase marketing plan set up by Microsoft/Bungie for Halo 3. Here is the brief rundown:
Phase 1: Starry Night campaign.
Phase 2: The Halo 3 beta.
Phase 3: Iris, the alternate reality game in the same vein as ‘I Love Bees’.
Phase 4: Merchandising tie-ins with various food establishments such as Burger-King.
Phase 5: Believe campaign starting off with the earlier reported on video.