I have seen AR used online in YouTube videos (though that was to access beta tracks/battle tracks, and I have no doubt other cheaters would be more malignant). I've even been cheated via wireless multiplayer- I overlapped my friend on Rainbow Road, and had a flawless race (finishing about 1-2 minutes before he did), but according to his DS I lost by a long way. Why? He'd applied cheats to his copy of MKDS beforehand, namely the "always-first" code.
Also, I think cheating in Brawl would be the same as in previous games; no-damage codes, or perhaps even infinite Final Smashes this time around.