Xbox Support's Twitter account is loaded with communications with customers about the problem, and the official support website at www.xbox.com/support is being bombarded by users. The company line is currently: "We're aware of the issue and it is being worked on. Stay tuned for updates."
Microsoft hasn't said if the intense rush of Modern Warfare 2 downloads and connections is responsible for the server issues, but folks trying to play the game are either simply unable to connect or getting error code 80150019, which is given to users when the console is unable to access their "gamertag" account information.
We'll say something here when Microsoft has an update.
Update: The matchmaking issues have been resolved but the Marketplace is still having problems.