Maybe I'm missing something, but I'm having trouble envisioning why a transparent phone would be useful. It would certainly be "cool", but that's really about it.

The keyboard projection part of the video game me an idea though: it would be really awesome if the iPhones could have built-in miniature projectors. That way, whenever you wanted to show something on your phone to a group of people, you could simply project the screen to the nearest wall or floor. It would have a real use case and make content more shareable.