GameApex has an informative article up which covers the topic of cheating in online multiplayer games.

The purpose of this series is not to teach people how to cheat, but to inform you all as to where we are in combating a problem that causes harm to the game industry. The extent of damage though is still up for debate. So I will limit certain details in an effort to not divulge too much information on how to cheat. It is also my intent to try to impress upon the gaming industry itself the importance of facing this problem sooner rather than later.

The State of Cheating in Online Multiplayer Games