I want to comment on what I perceive to be a common misconception about NFL rules regarding the use of helmets. I have heard it repeated several times by respected people that I follow that a running back can be subjected to helmet to helmet hits. This is not so.
"using any part of a player’s helmet (including the top/crown and forehead/“hairline” parts) or facemask to butt, spear, or ram an opponent violently or unnecessarily; although such violent or unnecessary use of the helmet and facemask is impermissible against any opponent, game officials will give special attention in administering this rule to protecting those players who are in virtually defenseless postures (e.g., a player in the act of or just after throwing a pass, a receiver catching or attempting to catch a pass, a runner already in the grasp of a tackler, a kickoff or punt returner attempting to field a kick in the air, or a player on the ground at the end of a play). All players in virtually defenseless postures are protected by the same prohibitions against use of the helmet and facemask that are described in the roughing-the-passer rules (see Article 11, subsection 3 below of this Rule 12, Section 2)
As you can see, wideouts get additional protection, but that protection does in fact extend to all players, even running backs. If the officials determine the act of the tackle to violent and unnecessary (see the debate here if you want to weigh in on that), they are allowed to flag the play.
Furthermore, if Collie had fallen to the ground and dropped the ball, the play would have been ruled incomplete for 'failing to complete the act of the catch'. Commentators say that two feet down turns him into a runner and thereby removes the basic safety protection afforded by the rules. This goes against the spirit of protecting players, and makes little sense. In the split second that his feet landed, he does not turn into a sacrificial lamb. If a catch is not 'complete' enough to withstand contact from the ground, it should not be deemed complete enough to remove the basic protections set up to ensure the safety of players.
As the rule above highlights, however, even if Collie was a runner, there is still no dispensation allowing for dangerous helmet to helmet contact by the defender. Note, that this rule also applied to the hit on Joseph Addai a few weeks back as well, as he was in the grasp of a tackler when hit.