Though the video isn't explicitly clear as to what it is teasing, the Xbox Live Marketplace offers the following clarification: "High action and deep mystery await players in this new Halo 3 campaign experience. Prepare to drop!"
The video ends with a mention of Halo 3--which hit Xbox 360 exactly one year ago today--along with the phrase "Prepare to Drop." It also hints at something occurring on 01/06/09, as seen in this screen capture provided by NeoGAF user Tapiozona, with many speculating that the new campaign may arrive on that day. BOOM video 973 /i>.