The policy change occurred 1 1/2 years ago (October 2010.) It was reported in Disney Files magazine, DVCMember.com, mentioned in the yearly vacation planner and numerous unofficial resources.
The dining plan can be added up to 48 hours prior to arrival with payment due at that time. The only time adding the DDP late presents an issue is if you are booking locations which require advance payment like Cinderella's Royal Table and some of the dinner shows. If booking those, you have to decide between paying for the DDP before making the reservation or make the advance payment on the meal and get it refunded later.