This is a pretty deap issue to dive into. It is based on what was to become Managed DirectX 2.0. We'll have a whitepaper soon for users who are familer with MDX and want to move over to using XNA. It should be available around the same time as the beta so I would look for that. In addition there's a whole lot more stuff that makes it more of a complete system for developing a game as opposed to just graphics API's
Thanks!