Almost half a year has passed since Major Nelson warned that cheating Xbox LIVE users will get their Gamerscore and achievements reset, and it seems that he has finally acted upon his promise/threat.
Earlier today, Microsoft punished "some" of the accounts they have identified as cheaters by resetting their Gamerscore to zero and revoking all their achievements. Cheating accounts have also been "tagged" with a message telling that they have been caught cheating.
Major Nelson noted that there is no way for those cheaters to regain their lost achievements but "earning them fairly, like the majority of the Xbox LIVE community does."
" There is no way to appeal a gamerscore correction. We only correct gamerscores for players who meet the criteria 100%.", reads the official faq. "Your best choice is to show the community you won't do it again."