jasonnc80 said:
One theory was that it involves the ODST's fight on Earth between Halo 2 and Halo 3. That might explain the date difference? I could be wrong because I don't follow Halo canon too closely.
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http://www.joystiq.com/2008/09/25/bungies-halo-3-campaign-teaser-your-theories/
This video basically confirms both of our theories 
Joystiq analyzes the trailer and comes up with the following (that we didn't already notice)
-You can see the Prophet of Regret's ship faintly in the background, meaning it's definitely set before the middle portion of Halo 2 (when the Chief killed him).
-The city is definitely New Mombasa, as confirmed by a sign that is briefly visible in one of the shots.
-They are indeed ODST pods (not Covenant like I speculated). They prove this by comparing them with images from the Bungie.net database.
Looks like this will be Bungie's answer to all the people who wanted to experience the Covenant occupation of Earth for themselves!







