Method, a group well known from WoW, has already beaten Skeleton King on Inferno difficulty! I am almost finished Nightmare.... Looks like I got some catching up to do.
They have only killed the Skeleton King at this point. Do note that up to the Skeleton king is only part of the first act and they still have lots of the game to go till they complete act 1.
Another point to be made is that this is a group of fully co-ordinated, skilled, dedicated and hardcore gamers who have been playing regularly since the moment of release. It will take quite a while longer for the average player and even most seasoned gamers to reach this point. That being said, it's amazing to see the speed in which they have reached this stage of the game, but remember that the hardest is yet to come...