Mpeg4 IS up and running StarzHD, HGTV HD, Food HD, NFL Network HD, and all HD locals with the exception of LA and NY are currently in Mpeg4.
There was initially some major quality problems with the Mpeg4 channels... but they seem to have worked most of those problems out. If you look at the thread regarding the SF Bay HD locals, you will see those problems detailed. But, Dish Network was very responsive to them and kept in communication with a few of us on the forums, and the problems got worked out. I still think the Mpeg2 channels are of slightly better quality, but the difference is negligible. When it comes to the locals HS quality, a lot of it has to do with getting a poor feed from the local affiliate. I know with the channels in the Bay the ones that had problems didn't even look that great OTA.