As someone who never watches boxing this whole thing looks like a giant clusterfuck to me, and I agree with whoever said that it only hurts the sport of boxing as a whole.
All I watched was this clip, several times:
http://www.youtube.com/watch?v=jD15cR4ke4w
What I see is Ortiz taking a headbutt and getting called for it. He apologizes once, touches gloves once, and goes back to his corner. You can hear the ref say "let's go" but there is nothing that looks like an official mark to resume the fight. That's only made worse by the fact that the ref is still looking down at the scorer's table when the fight is supposedly "in play."
Also I don't agree that the two touched gloves to signify the return to the fight. Ortiz went in to apologize again, quite possibly because the ref was still looking away and it was unclear that the fight had resumed. Mayweather bumped his gloves slightly but it looked more like an acknowledgement of the apology, trying to avoid the hug.
He then rocked Ortiz hard, and then again. People in here have acted like the dude gave him 5 seconds to get his guard up after the first punch. It clearly wasn't Mayweather's top punching speed but it was less than a second. Obviously Ortiz is a professional fighter but that first punch could very well have shocked him enough to keep him from realizing to raise his hands.
I'm not gonna argue about whether it's legal or not, as it seems to be, but I think that the ref really fucked up. The state of the fight was pretty unclear to me. That said, Ortiz caused the whole mess by headbutting. If he'd kept his initial flurry up, who knows, he could have eventually won. Probably not, but you never know. Ortiz then made it worse by attempting to apologize three times in the middle of a heated fight.
That being said, it was a cheap shot, legal or not. Even if the fight is officially not controversial and Mayweather won it legally, nobody should respect that kind of fighting.