I am a video editor with 2 HD edit suites.
The SD material you see on your set looks nothing like the material when it starts out with.
Your average SD broadcast material can be found in a few formats
DVCAM, BETACAMSP, Digital BETACAM, ect. (HDCAM and Digital Betacam can support True HDTV using an interface called SDI.. or Serial Digital Interface.)
There is an industrial, or consumer/prosumer version of HD called HDV.. it follows the same resolution(up to 1080i), however it is DV based, and has a much smaller colourspace and heavy compression.
In any case.. if you were to walk into an edit suite, editing SD material off of Digital Betacam.. you'd see detail and colour that *almost* comes close to HDTV in terms of general PQ (It's not of course, but visual.. compared to the end result on YOUR TV.. the source tapes or edit master are miles ahead, even on SD)
In fact, one of my first reactions.. seeing an HDTV off air broadcast on a consumer TV was:
" Wow.. that looks almost like the Edit Master!"
I'm convinced, if I hooked up a Digi Beta machine playing NTSC SD footage shot 16:9 into an HDTV set with a good scaler.. that even with the 480 upconversion, I could fool more than a few people into thinking it was HDTV.
What's my point?
Well, I guess SD can never, ever look like HD.. but, given the right circumstances.. you'd be really surprised how close it can actually get.