Transparent cellphone concept
<iframe width="640" height="360" src="http://www.youtube.com/embed/tkhWXspmiYE?feature=player_embedded" frameborder="0" allowfullscreen></iframe>
link
Should be saving lots of battery life by not using a backlight. Especially since its batteries will be really small...
I suspect the rear display could be used to provide a dark background if necessary.
And just think how much easier augmented reality apps will be to write!