This is very - VERY normal. Nothing to worry about unless you regularly view your plasma at 6". I have seen the same effect on every single plasma I have ever installed and it is definitely most noticable on gradient backgrounds where the entire background shifts from say - light blue to dark blue - or from black to dark gray. You see a definite dithering pattern as if the plasma can't handle all the colors it is receiving smoothly.
I have never nailed down whether this is in fact dithering from the source material, or if it is a limitation of plasma displays, but I have seen it on at least 100 or more different plasmas in all price ranges.