Researchers from National Taiwan University presented their latest project called NailDisplay. The idea is to have a display on your finger nail that can be used for several applications such as showing what's beneath your finger when it's obscuring your phone's screen (while typing, for example), and controlling screen-less devices. Of course one can also imagine a fingernail phone...
The display used in their prototype is a 65K color, 0.96" PMOLED (96x64). Obviously in the future one could use a flexible/transparent OLED that will simply be unseen when not used rather than the bulky device used in this prototype.
Back in 2010 Zikon showed their color nano-technology based electronic-ink display technology, discussing the idea of "color-changing" nair polish (this never materialized though). I'm not really in favor of wearable displays or computers, but this is one promising area for OLEDs.
