Here's an interesting, technical article on the subject of cheating in online games, and the challenges facing game developers in preventing it.
Some of these attacks seem very difficult to stop, and he doesn't really give a solution. For example, if there are cheat codes available in the single player part of the game, then the attacker can take snapshots of the process memory before and after using the cheat code, and figure out what parts of memory changed, and now they know how to achieve that same effect without the cheat code, by altering the process memory directly. The only easy solution is to remove cheat codes from the single player part, but that would cause an uproar from the lame pussy segment of the game playing population.
LOL YES thus 99% of the GTA San Andreas game beaters were cheat coders like myself LOL!!!