Poking around the XBox.com forums, I stumbled upon this post from Major Nelson:

[quote]

For all the players who have registered for and are competing in the Global Arcade Challenge, there is an update on the Small Arms competition. Unfortunately, due to technical difficulties, the Small Arms TruSkill Rankings Leaderboard was not wiped on 12/24 at 12:01AM GMT. To ensure all players have a level playing field, we are moving the competition to early January. We are in the process of working out the details and will post them later this week. When the new date is posted, all you have to do is play; all registrations will continue to be valid. Thanks you for your participation in the Arcade Challenge and hope you are having a happy holidays!

[/quote]

Considering how badly I had my rear end handed to me on Christmas day in this game, I'm not all that disappointed.

You can check out the post here.