ADDED: The WSJ has a spiffy description of the controversial end of the fight:
Deep into the fourth round, Ortiz, already in a hole against his more experienced opponent, tried to work on the inside. He drove Mayweather into a corner and sent his head into Mayweather's chin. Referee Joe Cortez jumped into the fray, stopped the action, and signaled for a point deduction on Ortiz.
They tapped gloves. An embarrassed Ortiz stepped forward to hug Mayweather – a move quite common after a foul in boxing. Mayweather leaned forward, as if to acknowledge the apology, and then plastered Ortiz with a short left hook. Ortiz flew back and glanced toward the referee in protest. As Ortiz looked away, Mayweather drove a straight right to Ortiz's chin and he crumpled to the canvas and was counted out.
Asked about hitting Ortiz as Ortiz went to embrace him, Mayweather commented, "First he does me dirty, and then he wants to be nice, then he wants to do me in again. You have to protect yourself at all times."
Most of the crowd erupted in boos after the knockout, and a great debate began. Everyone agreed that Mayweather's final punches were legal. Boxers are indeed instructed always to protect themselves....