As far as what I've heard (albeit its been a while since I've read up on XNA), it is possible to play games created with XNA over XBL, and you can download other games created by the community. Personally ignoring all my hopes of a true homebrew community where we can have XBMC and emulators, I think Microsoft have gone about this in a very smart way. To create programs like XBMC you would need to give programmers access a far larger command set and much lower level access to the core of the system, this is opening pandora's box for microsoft. This could open up the softmodding scene, running in a sandbox with a reduced command set gives Microsoft piece of mind, and satisfies developers who wish to create games.
I don't think all the people who say why can't we get XBMC, or give us "real homebrew" realise the risk this entails for Microsoft, afterall legal homebrew is just in its first iteration on consoles, they have to protect their investment. Sony have left themselves wide open to be hacked (with my limited knowledge of their homebrew system).
Just think of it this way people, essentially you pay $100 per year, to get unlimited access to community created xbox live arcade games, and if you want, you can help in the development of some titles. Forget about XBMC, its just a pipedream at the moment.