According to one AVSForum user, Comcast has started adding even more compression to their HD channels In order to squeeze more HD channels down the same pipe. "bfdtv," the forum poster, recorded MPEG2 streams from Verizon FIOS TV and Comcast. There's a clear, noticeable degradation in quality between the two.
Some would look at this as a bad thing—Comcast subscribers, for one—but I see it as a positive opportunity for the public to start understanding the nature of digital compression and demanding more quality from their content providers.
As you can see in the above excerpt shot from a Red Hot Chili Peppers concert, lead singer Anthony Kiedis is a much more gorgeous woman in uncompressed HD.
Comcast HD Quality Reduction: Details, Screenshots [AVSForums.com via Consumerist]