I preordered my N900 some time ago, when it was priced at $580 at amazon USA. Today, I wrote them just to ask whether I would be charged that price or the current price ($550) and he updated my order to reflect the new price "because the item was still not released".
He was not at all clear whether they would charge me this new low price if I hadn't asked. So, just a heads up for everyone that preordered at Amazon US.