From the XNA FAQ:
Q: Does the XNA Framework include the ability to use
A: The initial release of the XNA Framework on the Xbox 360 will not have any support for networking. We realize this is a big area of interest for game developers and are actively working on a solution for the next release.
Boo! What's that about? For the first time in my life, I thought of a game idea that actually sounded like something I'd like to develop, but it requires network support. I'm so disappointed.