Notice that the announcement for xbl trem was on April 1st. That date, of course, being April Fools day. On April 2nd their 'partnership' with Microsoft was broken off.
IT WAS AN APRIL FOOLS JOKE!
But, I was kind of disappointed. I actually signed into xbl that day to try to download it. I was really looking forward to it. I think that if tremulous was released on the X-bbox, it would be open to a much wider audience of people. Especially considering as Tremulous does almost no advertising whatsoever. I was really looking forward to trying out the control schematics and the new aliens and humans. It actually seemed like it could happen!
Well, I guess that just goes to show how well they pulled off this April 1st prank.
Good job Devs.