The current rule, as preposterous as it might sound, is that you
cannot touch the quarterback's helmet in any way for any reason. If I remember correctly, the rule change occurred either this offseason or the previous one. I've seen 15-yard penalties being called on good-natured slaps to the back of the head that occur after the play is over; it's gone to absurd lengths.
The irony, of course, is that, as I pointed out during the game, there were several plays in which it was obvious that the Lions should have been called for hands to Rodgers' helmet. I've seen several plays throughout this season in which illegal hands to the face should have been called on opposing players, infractions blatant enough to change Rodgers' momentum, but no calls were forthcoming.
It's a dumb rule all around, and I don't think anyone outside the NFL Competition Committee would seriously disagree. What's next? Personal fouls called for slapping another player on the ass?