The DU people told me I would get it and what the previous poster says is totally true, I didn't. They may just not understand what the rules are with the Disney Visa. I had a guest service guy on the boat phone and talk at length with the Visa people and he got the exact answer about the "group rates". Kinda odd that Disney Visa would advertise these things so extensively when in reality unless you book yourself with no other incentives you don't get the onboard credit.
DU had a block of rooms, which is why we got a better rate and better room, so I really can't complain about them, other than I wished they had informed me about the Visa thing so that I wasn't expecting it on board and didn't have to go through all that at the Guest Service desk.