Actually LCD screens can succumb to a form of burn-in. With LCDs it's called image persistence. If the same crystals have been activated for extended periods of time then they start to develop a memory for that state. The effect is very similar to burn-in. The plus side though is that its usually reversible by simply turning off the screen for a day or two. With phones it's not likely to ever be a problem because the screen spends more time off than on and gives the crystals plenty of time to relax between charging sessions. But I suppose if you left it in the cradle for a year without turning it off then you might have a problem. CRTs work by the inside of the glass being coated with phosphors and firing electrons at the phosphors, exciting them and making the emit light. If that same phosphor is continuously excited it will slowly lose their ability to emit light. Plasmas work differently but the burn-in happens the same way as CRTs. Plasmas use tiny neon/xenon UV lights to excite individual phosphors (in a highly simplified way). The same burn-in happens from continuously exciting the same phosphors.