There are long threads on AVSForum about this very topic. From what I've read, if the set has burn-in, it is irreversible. Running the "clear screen" setting will not fix things once burn-in has occurred.

The suggestions they have on the forum threads are to turn down the panel brightness from the default when you install the set since panel brightness seems to have an influence on burn-in.

LG specifically does not cover burn-in in their warranties.