Every game I watch I see at least one horse-collar tackle. Not every Cowboys game, but every game. Urlacher did it against Minnesota and got up laughing on Sunday. Twice this past weekend I was watching college games where the announcers pointed out a horse collar tackle and said that the NCAA should implement the same rule. Williams might have "perfected" it, but lets don't kid ourselves that it doesn't go on all the time.
From what I understand, what Urlacher did was not a horse collar tackle, per the rules. they few times I have heard it talked about, the key is that you are grabbing inside the shoulderpads, and then yanking them down. In Urlacher's case he had a handful of jersey, but none of the pads, therefore by the rules it was legal. I have seen many other cases in the Bears games where DB have gone inside the pads, but did not get called for it because of the way the players fell to the ground. If there is not the backwards jerking of the offensive player, it seems it will not get called.