For those in the US that have bought the game today and had trouble redeeming your X360 Online Pass code I have a status update. An issue with the authentication has been found which may be causing a message to appear along the lines of "code invalid" or "code in use".
Once you have entered your code close the game by heading to the dashboard and then reload FIFA 11. This may refresh the authentication of the code and then validate it.
The tech teams are currently working closely with Microsoft to get this resolved.
In the meantime you have a 7 day free trial you can utilise to start playing online.