I have written recently about my belief that the GM's position should be a reactive one, with most of the focus being on actively paying attention to the players and providing them with what they need in a scene. As the player outside the overall narrative thrust of the game, the GM is in a unique position -- required to provide generous moments to players, really, because the game is not about the development of NPCs. This is the biggest part of the GM's responsibility, to be there to push the right buttons at the right moments to achieve the effects the players are after.
In a game with a high-functioning group, this is what every player at the table should be striving for.
Does that mean that, in a group that's firing on all cylinders for dramatic play and sharing that juicy narrative control around, a GM isn't necessary? Well, perhaps. Let us say that a skilled GM is less necessary when there are enough others around who are taking up the slack. I'm not sure I'm ready to say the GM's day is done, if the players are good enough, but that may be my own gamer baggage talking.