Well they can do the Top Movie thing on your DVR, soI think it is doable. It probably would take a helluva a hard drive though. I do wish they would look into that though. They are missing out on a lot of possible revenue with those who don't have an internet connection. I think there are a lot more than we think that do not have it.
The "Top Movie" movies automatically download to the DVR through the satellite, but are not requested by the reciever itself -- D* just sends through a certain set of movies to all the DVRs.
For the VOD service to do that, the receiver would still need a way to tell D* which movies you want (Internet or forced callback). Then, the movies would have to constantly be available on the satellite for immediate download. To make all of the 1080P VOD content available through the satellite would take a lot of satellite bandwidth.