Wasn't Pierluigi Collina a referee in FIFA 2003? I clearly remember seeing him in that game. So why is it such a special thing to have him as "an unlockable" in FIFA 2005?

hmm...

didn't he retire from being a referee? it could be a reason

b/c he's the most famous referee in the world

It think you can unlock him as Pierluigi Collina himself. In FIFA 2003 and FIFA 2004 you wouldn't see the name of the referee. Maybe that's the reason.

And in FIFA 2005 you can only use him as a ref in International friendlies I think