Not before the game finally ships to retail, at least

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Xbox 360 buffs saw reason to believe Epic Games was in the process of preparing a demo download for Gears of War, thanks to some images from the official Xbox website itself. However, Epic Games VP Mark Rein denies his company has any plans to release a demo for the game on Xbox Live before it ships to retail November 7th next month, or even afterwards:

"Just saw the dog tag image with the "Microsoft points". It just isn't true. There is no demo coming out before the game ships and I have no idea if we'll do one after the game ships or not. Right now there are no plans to do one as I said earlier in this thread. I have asked Microsoft to take that image down."