Rebecca has a post looking at the word propitiation, its usage and a challenge to figure out another word that can fit in its place. Unfortunately (or fortunately depending on your bend in modern bible translations) I’m all for the actual word propitiation but I also readily admit that there might be a shift of usage depending on the Biblical author. Paul might be making an allusion to the OT mercy seat and John may be referring to what the mercy seat offering accomplished in a general sense. So whereas Paul might be saying "God publicly (instead of behind a veil) revealed the place where God’s wrath was turned away and satisfied in the person of His Son on the cross" John would be saying "Jesus Christ is the atoning sacrifice for the world, but for us believers especially."
Be that as it may, I’ll play and try to update (if not dumb down) the actual word.