Just got the PN64D7000.
"Wow, the PQ on this thing is stunning", I exclaimed. One of the best TV's I've seen in person, without a doubt. I quickly went to HD video after HD video admiring the finer details and rich, inky blacks. Unfortunately however my happiness abruptly came to a halt.
The brightness began dimming.
For those who don't understand, this TV's brightness, randomly and seemingly without reason, will dim to an absurd level, completely disturbing anything you're watching. All Eco settings out of the box were off, and so was the Motion Control setting, which I suspected was the other setting which had the ability to regulate on-the-fly brightness.
Fix thus far: I've been watching the TV for about an hour now with NO dimming.
1.) I reset to factory settings (As others have noted, hold exit for a few seconds, then press "OK")
2.) Went to 'Eco Settings' and then turned off 'Eco Sensor'.
3.) In picture settings I also turned off 'Motion Control' which was also on by default.
Oddly enough, even though they had both been off initially, turning them off after doing a factory reset has seemingly fixed things.
I'll update when I know more.