Microsoft has bigger plans for cheaters than simply resetting their gamerscore. VG247 noted that on the most recent Major Nelson radio podcast, guest Stephen Toulouse outlined harsh penalties for those who have glitched their way to high scores. "The bar is a little bit higher than that," he said in response to a question about resetting scores. "The other thing it does is, it puts a tag that you've been cheating on your gamercard. That's a pretty big Scarlet Letter."
Conceivably an offending party could just get a new gamertag, especially since after having their achievements reset to zero they're not allowed to get any of them again, even legitimately. But that would also force them to transfer over all of their friends, and Microsoft might have steps in place to prevent that too. This was triggered by a recent string of cheaters exploiting Gears of War 2, but Microsoft promises that score boosters won't be punished. It's definitely a stiff penalty, but they do say cheaters never prosper.