I read an article on this yesterday too. I think it had something to do with his foot or leg. I'll try to track it down.
pac seems to have sustained a foot injury specifically his right foot during roadwork. this still has to be confirmed though....
"It seems that the boxing world has received an unfortunate setback. A source close to the There Will Be Blood Fight Show revealed that Manny Pacquiao has just suffered an accidental injury during his preparation to face off against Ricky Hatton on May 2nd. According to the source, Pacquiao hurt his lead foot during roadwork, which would make it the right foot for the southpaw slugger." ----------------- Never heard of the site that posted this, so I'll wait for another site to confirm (or deny).
Someone close to team pacquiao said this on pacland "i was able to talk to atty jeng and he authenticated the story with michael koncz and the wildcard people--- this is not true... pac only had blisters in his right foot..."