I'm going to put it out there and say that, if you look at how Floyd has fallen off in recent fights, and how he struggled against Maidana (who in some ways fights like a MUCH less experienced and effective version of Pacquiao), this is not the total shutout for Mayweather that most seem to think it is. I'll put myself on the line here and say that it is not a big stretch for me to see Pacquiao beating him, and that the two are close to evenly matched given that styles can play as much role in the outcome of a fight as skill.
Mayweather does not like southpaws, as he loves to use (overuse, perhaps, in as far as you can criticise something he's made very effective) that lead right hand. If Pacquiao can stay on the outside, and keep a high work rate, I can see this going his way. Here's why:
Pacquiao is one of the most effective fighters I've seen in a very long time when it comes to punching from odd angles. He's an awkward little fucker and awkward little fuckers cause problems for defenses that do better against technically skilled (read reasonably orthodox lines of attack) fighters.
Punch output: Mayweather throws 30-35 punches/round and averages a 50% connect rate (18 punches/round). Pacquiao throws c.70 punches/round and averages a 43% connect rate (30ish punches/round)
Bare in mind Maidana landed at a rate of about 25% in his fight against Mayweather - and Maidana faded and lacked Pacquiao's experience, I can't see the former happening here.
Pacquiao is also hit something like 8-10% less than the weight class average. Not exactly a defensive slouch himself.
I read a great article on this the other day, and I'll see if I can dig it up. I was on the fence, but following it, I'll nail my colours to the mast and say I reckon Pacquiao will edge the fight.