An update to Modern Warfare 3 today is wiping the whole gamerscore of players downloading content from the Elite Drops 4, 5 and 6 which is now live from the Modern Warfare 3 store. I too have been a victom of this and giving out warnings not to download the Elite Content on MW3 or accept the Update.
Many gamers are having their gamerscore reset as part of a glitch with the game, this is not a gamerscore cheating reset, but a glitch which is telling your console you recently played the game on another console before it wipes out all of your game history and Achievements.
It seems the issue is wider than Modern Warfare with many players having resets during games of Battlefield 3 and Mass Effect 3, so there is a problem with Xbox LIVE servers at the moment. An update from Xbox Support has said the issue is being resolved and signing in and out of your profile today should bring them back at some point.