I read the Amazon pre-order guarantees you'll always get the lowest price if it drops between when you pre-order and when the game is released. So if you pre-ordered at $29.99 and it drops down to $19.99, then you would only pay $19.99.
Am I right? I rarely pre-order, and when I have that's never happened for me, so I can't verify it, but that's what Amazon says anyway.