Is it a touchdown if the ball breaks the plane of the goal line on the out of bounds side when a player is diving?

The ball has to be in the touchdown zone. If it is above the zone it is still a touchdown.

Take Troy Polamalu's touchdown on Dec,12,2010.

He dived, broke the plane of the goal line but he was holding the football above the zone so it counts as a touchdown. In some cases the ref might not count it.