Posted also in owners thread - wasn't sure which was most apt and where it would be most appreciated, if at all!!
Following on from the difficult to understand HDtvtest post (for me anyway), here's how I defeated the auto-dimming; hopefully this is clear and easy to follow.
Put the tv in standby mode, then press on the remote: INFO then MENU then MUTE and then POWER to put the TV on again. The TV will now start up and the service menu will apear on the left.
Scroll down to Advanced then press the numerals 0000 on the remote.
You will see a box with CE DIMMING in the second or third column along (there are lots of boxes), scroll (you cannot jump to it) to it then click ENTER (on the remote).
Then click on Standard or Movie then go left or right to select on or off.
Then power off the tv. When you start it up again, the picture settings will revert to default, so you'll need to record your settings before attempting this.
And there you have it, no auto-dimming. Beware messing around in the service menu though; unless you know what you're doing just jump in and out following the instructions above.
Hope that helps.