I have watched ABC and CBS HD and I do not see any visible difference. I have not watched 720p in its native form.There are not many sets on the market that show 720p I believe pannasonic was one but I beleive they ahve dropped it. It appears that 1080i will be the standard.
I have the 56H80 and the 720p to 1080i material I've seen looks just as good. 480p upconverted on the other hand...you can definitely tell the difference.