Just my opinion, but I think eventually all signals will go to MPG4. As more and more people buy 16 x 9 HDTVs, there will be less and less demand for the current MPG2 SD 4 x 3 signals. Once D* has all HD converted over to H20s and HR20s, it wouldn't surprise me to see them starting to changing over the older SD receivers to H20s, and all DVRs to HR20s. Then for analog 4 x 3 sets you would use letterbox mode with the A/V output. Like what we do now to view a 4 x 3 SD signal, putting it in pillar mode to view it on a 16 x 9 screen.
At some point, down the road, as the number of 4 x 3 sets declines, it just won't be cost effective to transmit the same signal in 16 x 9 and 4 x 3. Plus, it would be a big benefit to D*, having only one receiver and one DVR, for all customers.