I was browsing the 2K forums and read somewhere in their it said that Jerry Rice was the official spokesman of the game. That could mean he is the cover Boy.
Also judging by the new All Pro website it has a disclaimer about Johnny Unitas at the bottom. Kind of weird he is the only player with a disclaimer out of 250 or whatever.
I have a feeling the cover might have multiple players on it. As far as
PR goes in a game like this I would think it would be better to put up as many as you can on the cover. Everyone has different favorite players, so when someone looks at in the store, they might not have the same reaction seeing Barry as opposed to Butkus etc....