Microsoft looks into the future with their patent for a low-powered interactive secondary display, where the secondary display will sip very little juice in a rather similar vein as that of an e-ink device.
This particular display will show off various information which will depend largely on the state of the primary display. What you see above shows the secondary display taking up the entire back segment of a device that might be a smartphone or a portable media player (probably the former), where it shows off a clock as well as miscellaneous information, not to mention what looks like a camera embedded in the upper left hand corner.
Never mind if the device in question is in sleep mode, the secondary display will still show off the relevant information all the time.
It would be even more interesting if this particular display could be energy efficient enough in due time to support the use of gestures, although a low-powered display is a good start enough by itself without having to be too ambitious from the get go.