I saw "burn in" on a philips LCD a few years back that was being used at my workplace. it seemed that the pixels were almost stuck "open", allowing light to pass though. One coworker said that with a little excercise (ie. something like one of those plasma screen break in dvd's) the pixels would begin functioning properly again, but soon after we noticed, the screen died anyways, so we never found out.
In short, I have seen it happen once, but I've got no idea if it's common or permanent.