Gamespot.com said they think the game trailer is occouring in the future. Personally, my prediction is that the Large flying object the Chief was on at the end of Halo 2 crash landed, which is where the trailer picks up. There is debris everywhere, which could be the effect of a crash landing...
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Look in the background, that's the new mombasa bridge. You're in new mombasa after you've warped out, done delta halo and travelled back (possibly in time) on truths forerunner ship. When you go to slipspace in the atmosphere, it sets off a nuclear explosion, hence the nuclear wasteland he's walking though. IT's not the future, if anything it's the past ("i know your past, [u]your future[/u]")