My SXRD XBR is a first gen of it's type. It upconverts 1080i signal to 1080p, but does not accept 1080p through any means. Discovery Channel HD Theater looks better than real life, I kid not. I think the 1080p upconvert has a lot to do with that. I think also that should it you place a newer model next to it, one that accepts 1080p and of course gave that TV signal at that resolution, that it wouldn't look any better. Side by side, Blu-Ray for example I bet would look as good as any TV out there.
In closing I think if your TV displays a 1080p picture, whether it can get a 1080p signal or not, you are doing good.