They shouldn't even think about making this fight until both Manny and Juan take care of business first in their respective divisions. Manny still has Marquez, Guzman, Soto, and Valero lurking at 130, meanwhile, Juan has the likes of D. Diaz, Casamayor, Campbell, and Katsidis.
When Manny becomes the virtually undisputed 130 lb. champion (barring the practically worthless IBF strap, the only belts that matter there are Marquez' WBC strap, Valero's WBA strap, and Guzman's WBO strap) and Diaz becomes the undisputed 135 lb. champion (his mandatory challenger is the well-deserving Campbell, and there are rumors that D. Diaz might make a voluntary defense against Katsidis before the winner of that fight takes on the winner of the Casamayor/Santa Cruz fight). Manny Pacquiao/Juan Diaz, if it ever happens, shouldn't happen until at least late '08 to mid '09.
If it happens, I don't think Manny'll be able to bring his power up to lightweight, but he'll still probably be the fastest fighter that Diaz will have fought up to that point in his career. He'll have to use that speed, because if he lets Diaz build up a head of steam and push him around the ring using his superior size and strength, it'll be a very rough night for Manny. In what should be an exciting fight, I'd predict Diaz winning by virtue of using his size and imposing his will on the smaller Pacquiao, who won't be able to keep Diaz off of him with his punching power alone. As funny as it sounds to say, the only way I see Pacquiao winning is by outboxing and outmaneuvering Diaz.
__________________ Israel Vazquez....Super Bantamweight Champ....Super Heavyweight Heart
This man is my hero.