I saw a feature request from a while ago (http://trac.adiumx.com/ticket/2089) and there is an Adium Xbox project (http://code.google.com/p/adiumxboxlive/) which seems to scrape the screen to get something, but doesn't seem to have any sort of plugin as of yet....
I was a bit surprised that MSN did not seem to use their standard chat protocol for it either, but perhaps they are just using a slightly altered protocol?
Anyway, I would like to see this, it would be nice to chat with people online occasionally and also know if they are inviting me to game potentially. Even if my xbox is not on....