it can and will burn in.
I had that on my 42pw23, when i first got it, i had it set to auto aspect switching.
After about 7 months i had two darker bars down the sides when watching a widescreen signal...........no one else could see them untill i pointed them out.
Some how i messed around with the settings and now when watching sky or video, it stays in widescreen.....it only auto switches when watching DVD's.
Cos we watch Sky the most, 5 months further on and the darker borders are only just visable, if you look hard and at certain angles you can just make them out.
The mirror has nearly become uniform in brightness right across again.........
I would advise to leave it on widescreen, as most programmes are now broadcast in widescreen and only the odd 4:3 programme looks wierd, mainly the local news, but the simpsons look's fine....