There appears to be websites where you can download game saves to your Xbox 360 console so you can unlock achievements in a few seconds. In 2008 Microsoft punished most serious offenders by resetting their Gamerscore and not allowing them to gain the same achievements again. One cheater, Aaron "ColdRaccoon" Mesec, from Murrieta, California told MTV Multiplayer "I started cheating, I guess, because I got addicted to gamer score completion".
There is a blog, Xbox Cheater Watch, which is dedicated to discussion about probable cheaters on Xbox Live. It has been pretty quiet this year, but you can still find blog posts about unconfirmed and confirmed cheaters. A cheater will get the text "They've been caught cheating." on their xbox.com profile. If you are interested in seeing real life examples, here are some:
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