Let's wait and see what Liberty Media does with the DVR in the near future They will have to pay either company a licenseing fee for the software and TiVo has had the more reliable software.
Keep in mind that D* STILL supports (as in keeping running) those old Ultimate TV boxes - think about how old THOSE things are & how long they have NOT even been made!
I'm pretty confident the D-Tivo's WILL be running for quite a long time...
When you hear people talking about the MPEG-4 switchover, kind in mind that's for HD programming. As disrich says, UTV boxes are still being supported so it stands to reason that the D-TiVos should also be able to receive SD programming for quite awhile longer.