"Xbox LIVE System update out today. No big changes just an update to default settings that will only be visible to certain users in the EU" -Tweet from Major Nelson.
So there is the official explanation but I dunno... the update seemed to take pretty long to download. Like, more than a couple of minutes. It doesn't seem like that would be the case if it was just some rinky dink update that changed a few default settings for certain EU users. One has to wonder if it actually has anything to do with the recent leak of Gears of War: Judgment.
Perhaps Microsoft updated the system security settings so that they can ban people illegally playing the game before its release (it comes out in March). They have already threatened pirates with lifetime console and account bans.