For what its worth, I have noticed on my c8000 that the black level shifts too at times. At first I thought it was my imagination when I was watching full screen material, but now I have confirmed it on a letterbox version of Top Gun I was checking out. The scene I saw it happen in was not even very dynamic, it was just two guys talking in a room without much change in scene picture at all. I noticed 3 very distinct shifts in brightness of the black bars in about 10 seconds, which occurred when I rewound it, 3 times in a row.
I tried this both with Eco mode both off and on, with the same results, in a pitch black room. Dynamic contrast was off, as were any other filters that I thought could have had an effect on this.
It really doesn't bother me, the shift is pretty minimal. It does remind me a little bit of the G25 I had, but that was much more frequent with the shifting.
So for those of you who are worried about LCE or that somehow the 2011 line has introduced floating blacks, I would not worry too much about it. I will be able to compare soon though when I have the d series set up.
There are variables of course, such as this is a 2.3gb compressed avi file. So I can't say that I am watching a perfect source. Didn't think that compression would affect the black bar brightness however, since this should be total black at all times. Time to put in Star Trek Bluray and see what happens.
Edit: Test with the Star Trek Bluray and noticed it right away, around minute 1:27. Closed my hands around the black bar to make sure it wasn't just my eye playing a trick on me. Definite shift in black level, or cell light, whatever you want to call it. The other variable is that I am playing everything through ps3 so I don't know if that enters anything new into the equation or not. Bottom line though, its not too bad and not nearly what the G25 was doing.