Interesting article about an incident in the Man U/Sunderland match. http://www.soccernet.com/england/news/2002/0904/20020904renniesafcmufc.html
this is definitely not the way it is taught in the courses, but seemed from the article to have been the "right" thing to do in retrospect. it doesn't hurt that he is an imposing figure, but this definitely should be the exception, not the rule.
NHL refs do this all the time, although are often instructed to first let them go at it to blow off steam (more like give the league ratings). i think it should be discouraged but its good preventative action especally with a known troublemaker like Keane. Refs in pointyball also try to restrain fighting parties but useally with less success...i dont see the problem with this in futbol as long as there is no execessive force used.
I don't think a ref should ever touch a player, especially from behind, or when he is 'fired up'. If the plater turns and gives the ref a smack (not knowing it is a ref) - he will be in even worse trouble than before. Interestingly in Aussie Rules (of course a lot more rough & tumble than soccer), the umpires now try to restart play and keep things going while the fight is continuing. it's amazing how quickly the players stop fighting when the ball is in play and heading towards a goal.
At 5' 8", I guess I'll still need to rely upon a full-volume blast of my Fox 40 to break up fights. That usually brings them to their knees if you can get within a yard of their eardrum!