YotaPhone 2 is the latest version of this tech. Basically, the phone has an e-ink screen on the reverse side with the normal calling, texting, and basic web functions visible to offer massive battery conservation. You can mirror anything on the front LED screen to the back, but obviously static stuff with lower refresh needs works best. No official word on a US release, but rumors are that something may be coming this year.
Anyway, I just thought this was an interesting idea, and a pretty cool solution to Android battery life sucking so hard.