I noticed this awhile back.
My HD channel used to take up the entire screen. Now, it's only about 2/3rd's of the screen, with black bars on the top and bottom. I can change my aspect ratio on my tv to zoom to get back to full screen, but the quality isn't HD anymore.
I find myself watching the regular version (mainly espn and espn2) just because it's full sized.
this is comcast HD.