In general Xbox 360 games are region free. There are some restrictions though. Different regions have different video formats so a copy of the game from one region may boot up, but the video may be distorted or not show up at all if the video format is incompatible with your TV. Also the online portion of the game may not work properly as the game will try to connect to the server in the region that it's set for.
This may cause you to experience lag in multiplayer matches. Another issue to be aware of is that game updates release at different times in different parts of the world. This can cause your game version to be out of sync with your friends if your friends are playing a version of the game from a different region than you.
Finally while the game itself is not region locked any DLC (Downloadable Content) for the game may be. If you purchase DLC for a game from another region it may not show up due to geofencing. Geofencing is the practice of locking DLC based on your IP (Internet Protocol) address. For instance if you live in the UK and have a US version of the game you may end up with the US DLC. Since your IP address would be a UK IP address the DLC would be locked to you even though you have a US verison of the game. There is no way around this.
Please note that Activision will not exchange your copy of the game for one from your region. Importing games from another region is not supported and is done at your own risk.