The Map Pack for MBU that was rumoured to be finished and in the hands of Microsoft more than a month ago seems to be no more.
Pat Willson, one of the developers for Marble Blast Ultra, has just posted on his blog (the fitting angrydev.com) that due to the fact that "...we are unable to come to an agreement with our publisher for the Marble Blast Ultra update." the update is dead.
The interesting thing is that according to the xbox.com MBU page, Garage Games is both the developer and the publisher, so I can only imagine that the publisher in question is Microsoft themselves.
It's a shame to see so much hard work go down the drain, as well as the revitalization that this update would have meant for the game. Hopefully the two parties will get this resolved and we will get the update, but at this point, I wouldn't hold your breath.
You can check out the full post here.