Epic Games forums states that the Lancer and Hammerbust items for Xbox Avatars will not be available starting January 1, 2012. Those who have purchased the items prior to the new year are in the clear and can keep them.
The post notes that the removal of the Lancer and Hammerburst are part of a new policy going into effect for "gun-like avatar items" on the Marketplace. It would appear that this is just the latest addition to the list of things Avatars can't have or do.
We've contacted Microsoft to receive a comprehensive list of "gun-like" items being removed.