There has been no ending date announced, but since they just released the plan to DVC members, I'm sure it will be around until at least the end of the year.
Even though my hubby and I have a DDE card, we have a split ressie in May so we're going to try the dining plan at that time. We may decide we like it better and not renew the card in October.
Booked it for November. Love the DDP! Call MS with your confirmation number. Tell them you want to add the DDP. You pay for it when you get there. You have 5 days before check in date to change your mind and cancel the DDP.