Thanks for the tip, I saw that on the L925 at the distributor, my L920 shows the screen burned as Chris' picture above but not just in the top of the screen but also in the middle. I believe the screen won't fix with your trick but I will try it anyways.
Tony
You have to differentiate a few things. The phenomenon called burn-in on screens and displays is dated back to old computers monitors, where often displayed, static images actually "burn-in" on the display. When they show something else, you'll see a ghostly image of the burned-in subject, most likely icons, status bars or in Windows Phones case Live Tiles and virtual keyboards.
However current modern IPS displays, used on the 920, 1320 or 1520 for example, don't have such an issue. If they display such a ghost image, you can try the method mentioned above, by displaying a pure white image for a couple of hours.
Don't do that with a device featuring an AMOLED screen though! Both the 925 and 1020 have such a display. For this technology, imagine every single pixel as some kind of candle. The brighter it shines, the faster it burns down and loses brightness. This is the reason why you can't cure such ghost images on an AMOLED device: if you display a bright white image, the whole pixel matrix starts burnings down, however the already more degraded pixels burn out even further - it's no use curing the hardware. Only way to fix it is by sending in the device for a warranty repair. I don't know how Nokia handles it, but they should replace the screen for free.
Last but not least the shown and described issues in this thread aren't classical burn-ins, it looks like the display got too hot and the matrix got somehow damaged. I don't think you can fix it yourself and would send it back to Nokia for repairs.
It is odd to see how many 920s seem to suffer from such an issue, it might me caused by hardware internals which got too hot for whatever reason.