My ten penneth:
Well, 'Battlestar Gallactica' S1 looks like frack on HD-DVD, so that's not a good start. Supposedly shot on HD, it's not a good advert for the format. The remastered 'Star Trek' Originals, however looks beautiful on HD-DVD, because, as Ms Newsome says they were shot on 35mm.
I watched 'Band of Brothers' in HD on Virgin and they looked pretty good, although given the amount of picture processing, desaturation and grading involved, I'm not totally sure a good upscaled DVD is THAT much worse.
Interestingly, I saw a demo of 'Thunderbirds' in 3-D (without glasses, no joke....) from a Blu Ray source in Cannes a week or two back and it looked pretty terrific. Especially if you really WANT to see the strings!
You'll never see the VT-produced 'classics' on HD (like er... 'Only Fools') because there's simply not the level of detail in the original, probably two-inch, masters. Your HD tv will do as good a job of upscaling the picture from DVD or broadcast.