There's a great post on the AVS forum about Comcast HD Quality going down, featuring screenshots and analysis.
A few months ago, I switched from Comcast to FiOS for more HD channels and a lower monthly price and I did notice that non-HD channels definitely looked better and HD channels were about the same. I have wondered how Comcast keeps playing catch-up with FiOS and the Dish Network -- in my last month or two of having Comcast it seems they were adding new HD channels every month. I know fiber has a ton of bandwidth available, but I wasn't sure how Comcast was figuring out how to stuff more channels into their old coax lines. Looks like according to this AVS forum post, they're reducing the quality of the HD channels to fit more channels in their available bandwidth.