Got an offer today in my account for $100 for spending $500 on a Norwegian Cruise Line reservation -- it is available on all my membership rewards earning cards but not SPG (which is the only other Amex I have).
We have a cruise this summer for which final payment is not yet due, but after registering my cards, I called up our TA and asked them to charge $500 toward the sailing on one of my registered cards, and I received an e-mail that it credited. So it appears you can use it for existing bookings in addition to new bookings -- the terms and conditions were not very clear on that.