Today Gowans007 on NeoGAF 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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