Bungie and Microsoft posted a video to Xbox Live that was kind of a making of the trailer kind of feature. (If you haven't seen the trailer, see it in all of its high definition glory.) They teased some things about the story, showed the live orchestra that did the music, and made a strong point that what you see in the trailer is rendered in real time by the game engine, and it's what you can expect to see. Awesome.
I need to find a plot summary of the first two games. It has been awhile and I don't remember the specifics.