My feeling is, since DirecTV has announced that they are no longer going to use NDS as its encription supplier that the 721 could have support for the encription scheme that DirecTV wants to use.
I do believe that the cards themselves are very simular between DirecTV and Dish (although they are programmed differently) so in doing this they could have the 721 support both.
It would save dish some money if they waited instead of swapping out someones 721 a few months more down the road.
The reason I believe they have not come to a final decision on hardware such as Hard Drive size is because they are sitting back waiting to see whats coming down the road.
Again smart move, but I still want my 721.