Examples that are not considered cheating (but not limited to these):
Boosting / trading kills, wins, etc
Switching your timezone/language
Offline gaming in general (provided the achievement doesn't require online access)
Offline gaming to avoid a title update
Reverting a game to a specific title update
Changing your Xbox date/time while offline (You Don't Know Jack / Batman)
Using a turbo controller to automate grinding
*.* seriously??? I would like to receive a confirmation :D