According to Xbox Live Marketplace description, this appears to be a new Halo 3 Campaign Experience.  Look for more info as it is released.  Video & pictures from the video can be found below.

*UPDATE*  A Joystiq reader found a date hidden within the trailer that reads "01/06/09".  Could this be a true release date, or a teaser truly teasing us. Will update with info in the near futire

Any speculation on what this could be about?  Leave us a comment below!