The way OLEDs get burn-in is a bit interesting. Its not burn-in as such, it's more like 'Reverse LED Fatigue'. As the LEDs get used, over time they begin to get 'lazy' and start to lose brightness. Now if you have an area where the LEDs aren't used, like the navigation/menu bar on the Galaxy Nexus, and you watch a full screen video, the space where the navigation bar would have been is slightly brighter than the surrounding screen, giving the appearance of burn-in.
Sent from my Snapdragon.