The entire rule from the rulebook at NFL.com:
http://www.nfl.com/rulebook
Article 7: Players in a Defenseless Posture. It is a foul if a player initiates unnecessary contact against a player who is in a defenseless posture.
(a) Players in a defenseless posture are:
(1) A player in the act of or just after throwing a pass;
(2) A receiver attempting to catch a pass; or who has completed a catch and has not had time to protect himself or has not clearly become a runner. If the receiver/runner is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player;
(3) A runner already in the grasp of a tackler and whose forward progress has been stopped;
(4) A kickoff or punt returner attempting to field a kick in the air;
(5) A player on the ground;
(6) A kicker/punter during the kick or during the return (Also see Article 6(g) for additional restrictions against a kicker/punter);
(7) A quarterback at any time after a change of possession (Also see Article 8(f) for additional restrictions against a quarterback after a change of possession);
(8) A player who receives a “blindside” block when the blocker is moving toward or parallel to his own end line and approaches the opponent from behind or from the side, and
(9) A player who is protected from an illegal crackback block (see Article 2).
(b) Prohibited contact against a player who is in a defenseless posture is:
(1) Forcibly hitting the defenseless player’s head or neck area with the helmet, facemask, forearm, or shoulder, regardless of whether the defensive player also uses his arms to tackle the defenseless player by encircling or grasping him; or
(2) Lowering the head and making forcible contact with the top/crown or forehead/”hairline” parts of the helmet against any part of the defenseless player’s body; or
(3) Illegally launching into a defenseless opponent. It is an illegal launch if a player
(1) leaves both feet prior to contact to spring forward and upward into his opponent, and
(2) uses any part of his helmet (including the top/crown and forehead/”hairline” parts) to initiate forcible contact against any part of his opponent’s body. Note: This does not apply to contact against a runner, unless the runner is still considered to be a defenseless player, as defined in Article 7 above. Note: The provisions of (2) do not prohibit incidental contact by the mask or helmet in the course of a conventional tackle or block on an opponent.
Per 7(a)(2) they were right not to flag for the hit on Wallace (where this entire conversation started) because he was now a runner, and no longer qualified as defenseless under the rule above. If he did not try and get up then there might have been a flag, but he clearly was doing so.
It's not like this is some kind of James Harrison-like haymaker with his helmet kind of hit either...
I await your quote Jimbo on where I've flip flopped on a rule because it went against my team. (maybe I'll just put this in my signature and link to your post)
lol 
