I think the Mosley fight is just a warm up for Mayweather. Mosley is a little bit older, and Mayweather is gonna be quite a bit faster.
Money-wise, look at it this way. If Mayweather fights Pacquiao first and then Mosley, he has nothing to gain. Pacquiao has more of a chance to beat him than Mosley does. He'll make a ton of $$ fighting Pac, but if he loses, he stands to not make as much money fighting Mosley.
Now if he fights Mosely first and wins, than he stands to make a good amount of $$ from those people who have been waiting to see Mayweather v. Mosely, and then make even more $$ with Mayweather V. Pacquiao.