DVDs can look very nice on a HD set. I have a 65" 16:9 set that has been ISF calibrated and I also own a true 720p projector (Infocus 7200) with a 119" 16:9 screen.
Make sure you have at least a good progressive DVD player if you are watching any DVD on a HD set. It does make a difference.
Better than that, there are some players that upscale the DVD to 720p or 1080i. They look even better than progressive players. Bravo D1 is an example.
Going from DVD to HD on a smaller set (65" or so) is not that big of a jump. However, you can see BIG differences going from DVD resolution to something that is in native HD on large screen projector set ups. I actually now have a HTPC array of hard drives with over 2 terabytes of storage...just for HD material. Got to love newsgroups!
Scalers do work but anything that is native HD will always look better.
I noticed a big difference between Mother's DVDs and all other ones that were released as well. Why? His has more resolution since it was from a PAL source. Scalers and Progressive players can only do so much.
It's all about the source.