You know what's wierd, I recently had an idea with my television, since on HD mode, my TV stricltly displays what it's fed I can't make any adjustments to it other than color and brightness on the DVI port. I ran a second S-Video cable to one of the other ports, now S-Video is what 480i? Also my TV has additional settings that can clear up SD signals for some reason, like make it seem more progressive scan than interlaced, and auto-adjust sharpness, in addition to auto-adjust the image so that it's widescreen, but cropped to not look as streched.
I actually get better PQ from doing it this way than watching a SD channel upconverted to 1080i on my DVI port. Maybe upconverted 1080i amplifies imperfections? All I gotta do is his channel up/down to go between the two ports and the improvement is very noticeable.
On my 36" Standard TV all channels look fantastic and better than Comcast's cable by a long shot. Again it's S-video cable, for a TV that's best possible resolution is S-video. Also the screen is smaller so imperfections are less noticeable.