There's a new concept mobile phone called 'second-life' that uses a 'double-display' which is an OLED display on top of an E Ink display, and both are transparent. You can switch between the displays: so when power is low, you can use the E Ink to conserve battery. A nice touch is that if you're using the E Ink display, the degree of transparency of the display shows how much battery is remaining. The more transparent the display, the less battery remaining.

Second-Life OLED/E Ink phone concept photoWe're not sure if it's possible to create a transparent E Ink... but this concept design is nice in any case...