Well, in my curent group, we have one person who I have to watch the dice rolls (or have another PC looking over his shoulder), but he is a 14 year old kid, so with a bit of luck, that will improve with time.
In a previous incarnation of our current group, we had one person who constantly cheated, on a large scale. We are talking about re-writing his character sheet between sessions, and adding one to all of his stats. Potions and scrolls mysteriously doubling in number, only recording half damage, buying his own copy of the module that I was running at the time, making up a vampire character with 45 freebie points instead of the normal 15, min-maxing to the nth degree by twisting rules etc etc. It actually took us quite some time to realize, and we no longer game with him or see him socially (we realized how much of a *not nice person* he was in general at about the same time).