It really does seem to depend on the channel. Since I got my new 46" TV a couple of weeks ago, I've noticed that there is a big difference in PQ from one channel to another. Some Star Trek reruns on a local channel looked just fine in SD. One of the worst seems to be NFL Network. I've been appalled at how fuzzy some of the video highlights look. The NFLST shortcuts, on the other hand, don't look too bad.
I think DirecTV is overcompressing a lot of the SD stuff. Before I got my new HDTV, I noticed that the picture was soft on many of the NFLST channels, even on my old 30" CRT TV.