We booked onboard the Magic March 15 cruise. My dummy booking was the Wonder sailing noted above, and was marked as placeholder on the slip that I received from the onboard booking agent.
I received an email about this 'placeholder' booking advising me of the changes to the system. (And incidentally, I think it's a great idea that will no doubt clear up Disney's inventory management and understanding of who actually intends to sail on any given date.)
I have another reservation as well, which was an OBB that was transferred to another date. I did not get an email about that one. Presumably since I can't move it again, they don't need to advise.