Display vendors have been working on flexible displays for some time. I saw them at the Society for Information Displays (SID) conference I attended in 2011. Since then, work has continued, slowly, but surely.
At this year's SID show in San Diego, Nokia showed off some of its latest advancements, and they were impressive. This is the portion of Nokia that Microsoft didn't acquire, and too bad, because it would have been a great pickup.
Nokia, in conjunction with Semiconductor Energy Laboratory Co. Ltd and Advanced Film Device, showed a display that can be folded in two or three while displaying video. When fully unfolded, the displays are 5.9-inch OLED panels with 1280x720 resolution and a pixel density of 249ppi. That's a bit short of the 400+ ppi people are used to with smartphones, but you can't have everything at once.
Bent, curved or folded displays have always been a challenge because of the glass, not the display itself. The companies got around that by eliminating the glass portion. The display is made by forming multiple layers (release, sealing, color filter, TFT and OLED layers, in that order) on a glass substrate. After the layers are sealed and attached, the glass substrates are peeled off and replaced with flexible substrates.
By doing this, the color filter, TFT and OLED layers are sandwiched between two sets of a sealing layer and flexible substrate. However, the flexible substrate can only take so much. SEL estimates that the display can be bent more than 100,000 times. After that, it will likely break, just like if you bend any piece of plastic or thin metal back and forth enough times.
The book-type and three-fold displays can be bent up to curvature radii of 2mm and 4mm, respectively. What that means is the book-type display, which is folded only once, can be folded pretty much in half and there will only be a tiny bulge at the fold/curvature point.
As expected, there is no release date for this technology.