I'm sorry if this has already been posted, I didn't read all NINE pages of this thread. Steve Mazzagatti did an interview about a week ago where he talks about strikes to the back of the head - his #1 "pet peeve". It comes about 8 minutes in.
Jarry Park - The Coliseum Of Combat Sports Interviews » Blog Archive » Steve Mazzagatti Continues To Lay Down The Law
He warns fighters about doing this BEFORE the fights back in the dressing room. He tells them that he will not warn them during the fight, but will immediately take a point off.
I'm not sure if I agree, but it's much more effective than simply shouting, "Watch the back of the head!" over and over again like every other referee.
I admit Lensnar's got a case for feeling screwed, but punching the back of the head is probably the least enforced rule in MMA.