"Item 1. Player Going to the Ground. A player is considered to be going to the ground if he does not remain upright long enough to demonstrate that he is clearly a runner. If a player goes to the ground in the act of catching a pass (with or without contact by an opponent), he must maintain control of the ball until after his initial contact with the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete."

I hate that rule, but I think the referees applied it correctly in the Patriots/Steelers game.