While I have never been accused of cheating (because I steer clear of multiplayer games in general), I have had friends who not only have been accused of it, but reported for hacking, as well. The kicker was my friend had a relatively new piece of equipment that allowed for floating above the ground, and they were accused by no less than 3 different people as soon as she'd gotten it that week.
I sometimes notice people who do exceptionally well and while I DO think about "gee, how'd they do that" it doesn't always turn to cheating. Usually, I suspect people of exploiting cheap tricks, glitches, etc... which isn't really cheating, anyways. But more to the point.. in an offline game, I fully endorse cheating because it's your game, play it however the hell you want. But yeah, don't do it online, because it'll just piss people off. Like in Saint's Row 2 how a cheating player can effectively screw up other people's games.