If you're interested in Yotaphone's crazy, E Ink-packing double-screen system but don't want to give up your iPhone, we've found a case at Mobile World Congress that may have the answer.
The InkCase is a slim, light sleeve that somehow fits an E Ink display into what feels like a regular case. Info from your phone is squirted to the rear screen via Bluetooth 4.0, so it doesn't use much power and won't clog up your Lightning-hole.
Unlike the Yotaphone the InkCase will only display information from the three specific apps made by its manufacturer, Oaxis, so you can't currently read a book on it (the iPhone version is a bit small for that, anyway). There's also a larger version for the Note 2, and Oaxis will be bringing the cases to Europe in a couple of months.