Major Nelson has a blog post referring to a new approach from MS against people using save games or other tampering to boost their gamerscore.
They're resetting the score to zero, and the achievements will never be obtainable again.
From the article: "If you decide to employ some nefarious techniques to artificially increase your Gamerscore or obtain achievements by manipulating the Xbox software without playing the game, bad things will happen. What type of bad things? For one thing we may remove the entire Gamerscore for an account that contains manipulated gamerscore. Let me be more clear: If you use tampered gamesaves to boost your Achievements and Gamerscore, you may sign in to Xbox LIVE one day and discover that we have removed your Gamerscore. Once we do that, you will never be able to re-earn those achievements on that account. Even worse, you might have your account and/or console banned from Xbox Live."
Link: Example Gamercard