After reading all these replies, I'm now really curious why my SNES wont display on my HDTV. Everyone else seems to be having good luck.. Maybe there is some setting I need to mess with?
My HDTV is a run-of-the-mill LCD, with 1080p and 120hz refresh.
Keep in mind that old light gun games (the kind that make the screen flash) will not work at all on modern HDTVs. Aside from that, with the right HDTV and settings most retro stuff can look pretty good.
I've heard that light gun games won't even work on newer CRT's - you have to use the older CRT's with the screens that bulge out, and not the newer flat screen CRT's.