I have heard of this happening more and more lately - be glad that you were accepted, if you were not they would not lie to you. The call could come any day, believeme, I have been there.
I have had three different cruise contracts and two of them were "last minute" out of the clear blue sky. I was actually told I would be boarding on a certain date and was called up early both times.
This is a different economy, though, and word is that people are not leaving their cruise ships jobs as fast as they used to - they don't want to take a chance on not being able to find other work on land. Plus, fewer ships are coming out now than before.