Gizmodo points to a Youtube video and New Scientist article which detail and demonstrate a technology called "LucidTouch" developed by Mitsubishi Electric Research Labs.
The prototype video demonstrates a user using the back of the device as a touch surface to manipulate objects on the screen. The prototype uses a mounted video camera to provide visual feedback on screen as to where the users fingers are. By using this "see-through" interface, it attempts to solve the issue in traditional front-touch screens where fingers may obstruct the view of the screen itself.
Apple has also been researching this technology which was revealed in a patent application from January 2007. The patent application was titled "Back-Side Interface for Hand-Held Devices". Apple's implementation did not go so far as to depict the users fingers, but instead opted to display a pointer or cursor to show where contact was being made.
