We have been through this question before on the boards - on second thought there probably aren't that many topics we haven't discussed before. The search function works wonders.
It happens very very infrequently on lcd's, I mean you really have to be working at it to get it to happen; but to cure "burn-in" ( which isn't really burn in, its more like stuck pixels ) throw up a bright white image and leave it for a few days. That should fix the problem.
Ed is correct on plasma - they suffer from burn in much worse than lcd.