I give up trying to find it, but someone at another board I post at linked this article where this guy did a study and listed the positions on the diamond from hardest to field to easiest. He basically stated that you can move right on the list throughout your career, but not the other way around.
I remember catcher was first, shortstop next, and first base was last.
I can't remember if LF came before 3B or not, though.