"Since you don’t need an Xbox 360 development kit to start coding Xbox 360 games, Microsoft has published a document targeted at unsigned Xbox 360 developers which reveals some interesting info.
Developers who are interested in writing games for Xbox 360 can be productive without having an Xbox 360 development kit. This paper describes hardware and software that can be used to emulate Xbox 360. It also includes recommendations for programmers wanting to get a head start on Xbox 360 development." [more]