I don't think that the MPEG-4 decoder chips became available in bulk quanities until the end of last year. So they really haven't been put into any of the STBs yet.
Besides, Dish has been saying that this will be a slow transition. The best guesses so far have been that first it will be just the New HDTV channels which will be Mpeg-4. Then the existing HD channels will follow after some period of time. There's only about 8 of them, so thats relatively not a big deal. The the SD channels will begin to convert.
By that time, the MP4 chips will be a few years in development and will be so cheap that it will make sense for Dish to be able to offer to swap out the millions of SD STBs that they have. l