Hope this helps, I have the 55-LB6500 and had exactly the same issue as in the video at the top of this thread.
While I could not disable the dimming entirely, I did find a combination of settings that made it almost unnoticeable, try this:
PS3 connected via HDMI1 - should work for other HDMI connected devices though on any port.
From the settings menu, choose Picture mode and select Game, then go to Picture Adjust
Backlight 61, contrast 73, Brightness 59, Sharpness 15, Colour 55, Tint 0, Colour temperature C20.
Now go to "Advanced control"
Dynamic Contrast : Off
Dynamic colour : Off
Gamut - wide
Super res: Low
Gamma : Low
Now go back one level and select Picture Options:
Noise reduction : Off
MPEG Noise reduction : Off
Black Level: High <------- I think this is the one that probably makes the most difference
The result in my case is that the blacks are not as black (have a grey tint), but I much prefer that to not being able to see anything in dark scenes.
I generally watch 1080 content or games and am in a well lit room so you results may vary, this is definitely not a recommendation on calibration, just something that helped fix my dimming issue.
Good luck! let me know your results or suggestions.