Activision and Treyarch must be thrilled: the Xbox 360 beta files have leaked on the internet and can be played by anyone with a JTAGGED Xbox 360. Some players have already streamed new content on Justin.TV and posted pictures on different forums. I've downloaded the files myself and they seem to be legit.
Unfortunately the FastFiles (.ff) are compressed with a new algorithm and nothing can be extracted (models, pictures, stats, etc.) at the moment. Here's a picture of the menu.