Let me first say, I have not purchased any non-HD content when HD was available since I bought that first HD-DVD player... what 3 yrs ago now? 4?... time flies anymore.
My quick test was of Ratatouille SD, Lost HD & SD, Stargate Universe HD & SD - all iTunes purchases. I've watched all these on a 58" and/or 65" screen. Even my wife can tell the diff between SD & HD now on most shows, although our current TV upscales amazingly well. I'm watching from a distance ranging from about 12-18" and sitting in a dark room.
Ratatouille on the big screen is definitely SD and it shows. Number one, the clarity is just missing. It looks great on the iPad.
Lost: Looked at lots of bright scenes - blue sky and ocean, orange explosions, greenery of forests... The colors are gorgeous... On both SD & HD. Just in a quick comparison, scene for scene, I can't see the difference. Maybe if I paused two different units at the same place and compared, but even I am not that picky.
Stargate: Looked at dark, black scenes with a lot of shadow. Still, sorry, can't see it. Space battle sequences look great. My guess, without directly comparing, is that there is some, and maybe a fair amount of shadow detail missing in either one of them compared to the big screen.
Bottomline: I'll continue purchasing HD content as the primary place I'll watch it will be on a big screen. (Let me also say, iTunes HD does not compare to B-R.) But, it sure does appear that I can put a check mark in that 'Prefer standard definition videos' box to save the space on my iPad instead of just assuming I needed the HD versions.