I just spoke to a rep from Carnival. Here's the deal on the $459 rate below:
The deposit is $200. This is non-refundible, so if you cancel you lose the $200. Also, name changes on this option are not allowed so you cannot pawn your cruise off on someone else if you decide not to go. Obviously, this is risky for us who will be trying to manage work schedules around this. But if you're positive you want to get then this might work for you. The group rate has more security built in at a bit higher cost (like $80 more). More on the group rate coming soon!