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Gowans007 on http://www.neogaf.com/forum/showpost.php?&p=18305497&postcount=1 has reported that Microsoft is now giving XBox 360 Devs the ability to stop guide party chat from within online games.
Currently, players have the option of forming a private chat with players, but that will soon change.
To quote: "Call of Duty: Modern Warfare 2 will force players to decide whether or not they'll play the game and join the game chat, or choose to stay in a private chat and not play in the multiplayer game.
With titles like Gears of War, Call of Duty: Modern Warfare, Call of Duty: World at War, and many other online based multiplayer titles, the private chat feature became a way of telling other players of enemy locations after being killed."
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