MoP heroics - pretty much any typical setup with a tank, healer, and 3 DPS. Other teams that have 4 hybrid dps/heals and a tank have also been successful. Heroics are doable if you know how to multibox well and group makeup is not a barrier.

Rated BGs - don't believe anyone has been successful in rated BGs, partially due to nobody wanting multiboxers on their rated BG team.
Arena - there has been success with 4 enchancement + pocket healer, and recently Ellay has had success with 3 DK, 1 enhance, 1 resto druid (all played by Ellay). We have not seen a dominating team in arena yet, likely due to lack of interest and the 5v5 bracket being dead. I am not aware of success in the 3v3 bracket, though I think some have won.

Go ahead and discard casual BGs, but I'm willing to wager 95% of the multiboxing population do more harm to the random BG team (AV and IoC excluded) than good. You need a viable team to carry your weight, even against PUGs. AV and IoC are easymode - anyone, including bots, can be successful.