Look at the situation just before the cross goes in:
http://img12.imageshack.us/img12/372/chievogoal.png
Pellisier is the Chievo player just outside the 'D'.
It is three on three, not six on four as only the three defenders and three attackers are relevant.
Zebina could have stopped the cross.
Molinaro goes with his man, as he had to do, unless he got a shout from Chiellini.
Chiellini, therefore, was the man who needed to get closer to Pellisier, but he was ball watching.
There should also have been better communication from either Buffon of Molinaro (as they were the defenders who could see the situation developing) to decide to go to more zonal marking, but that is an incredibly tough call to make.
There was only about eight seconds between Mellberg losing the ball and the ball ending up in the net.
Realistically, the main point that at which the goal could have been prevented after Mellberg losing the ball was with Zebina stopping the cross.
It should also be said, the movement from the Chievo forwards was very good to find space the way they did.