It depends on which Mayweather shows up. If he uses his feet, he will beat Margarito.

If he lays against the ropes a lot like he did with ODLH or Hatton, he could lose a decision because he will not deter Margarito and Margarito will win the fight on activity alone, even if 95% of his punches are blocked and parried.

Floyd is not stupid though and unless he comes into the fight rusty, I could not bet against him.