Last month Japan's Fujitsu and NTT DoCoMo previewed a prototype cell phone with an interesting and unique interface design: The screen is transparent and has touch sensors on each side.
The most basic application of this technology is that you can use your finger on the back of the phone, providing unobstructed viewing for the front of the screen. I was excited when Apple patented, a few years ago, a similar system that called for an opaque screen with touch sensors on the back. If they ever prototyped it, we'll certainly never hear about it unless it makes its way into a product. But now, watching the video below, I can see the drawbacks of such a system; the awkward angle of operation for manipulating the rear seems to cry "wrist cramp."
Ergonomics aside, a more advanced application of this interface is a sort of bi-level multi-touch. To see it in action, check out the Rubik's Cube thing the demo guy does around 0:50:
I also dig the Japanese self-effacing style of product demonstration, which is the opposite of, say, a Microsoft demo. Here the guy shows you what the thing does, then tells you what's wrong with it: Screen currently too small and too dim. But they're working on it.