Ive got a 55" X8505 and just like the chap JudgeC I pretty much only watch 720p or 1080p content.
I did watch a Sky on Demand episode of QI the other night which only have the SD copy available and I was surprised, I was expecting it to look awful, I reckon it looks about the same as it did on my 1080p LG 42", just bigger.
The bigger a TV the worse its going to look and also different 4K TV's are quoted as upscaling better than others.
I doubt the manufacturers care to much about SD though, if youre buying a big 4K TV then the assumption might be you're looking to watch 4K content when it arrives and also you already have a thing for HD anyway.
Im waffling a bit, just the first two paragraphs are relevant.