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In an episode of GameTrailers Bonus Round recorded at the Penny Arcade Expo, Brian Jarrard of Bungie revealed that Halo 3: ODST was completed way back in May and was held back to be released during the holiday season.
Jarrard goes on to talk about how strange it is to have been promoting a completed game for the past few months and mentions that the copy they showed at E3 was completely finished. It's an interesting discussion that can be checked out here. Thanks to Loot Ninja for the heads up.