I agree re. there being no noticeable difference using the more expensive HDMI cables.
I had the sales guys at Harvey Norman rig up a Full HD Panasonic instore to a Sky HD box with a monster HDMI cable and the result was the same as mine. The Ellen show on TV1 was very very blurry.
The only other thing apart from Sky compressing signals etc that I can think of is the cable coming from the dish to the telly although signal strength looks ok.