While I don't dispute the superiority of h264 vs. MPEG-2, the example of Dish's HD compression vs. Comcast's HD compression is off base.
The originating source material is broadcasted using MPEG-2. For the Dish system to offer HD channels in h264, they have to decode the MPEG-2 and then re-encode to h264. On the other hand, Comcast merely passes the MPEG-2 right through without. I would think that this only applies to OTA HD channels. As for "cable HD" channels (i.e. TNT-HD, HBO-HD, etc) I guess it would depend on how they receive the originating signal.
Also, DirecTV (and maybe Dish) has been known to transmit what is lovingly known as HD-Lite. What they do is take the original signal and down-res it to 1440x1080i (or worse), before encoding it to MPEG-2 (eventually MPEG-4). The results are that the HD channels tend to look very bad. When they go fully to MPEG-4, they may go back up to the proper resolution, or they may continue to down-res to cram in more channels.
Anyways, I think I drifted off-topic.
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