If you have already attained a copy of Modern Warfare 3 before its November 8th release date, don’t go online. Microsoft and Activision have not yet sanctioned online play of the game on the Xbox Live network. If Microsoft detects your Xbox online with a game that has not yet been approved to be online they might actually ban your account or even your Xbox from getting on Xbox Live in the future.
So play the game at your own risk, it may or may not happen but Microsoft is in their legal right to take action against early players. You have been warned.