In a discussion with a company recently I have found out what LCD burn-in really is... and some companies try to claim their problems are the same as what LCD has.
LCD has what is called image retention which looks very similar to burn-in. But, the problem is not permenent and can be corrected.
The problem is that when a LCD image is in the same state for days (not hours) then those individual LCD crystals retain a bit of an electrical charge that prevents them from properly twisting/untwisting all the way as they are supposed to. On a black scene, this appears as burn-in.
But, when you turn the display off you then should unplug the TV and let it sit for 24-48 hours so that all electrical charges from the display can disappate. This will allow the LCD pixel to fully drain its power and return to a zero state. The issue completely goes away and the LCD is really as good as it ever was. No burn-in at all.
Plasma? Nope - when you have it, it's here to stay - and it is ugly.