In an article recently published by researchers from Carnegie Mellon University and Disney Research, a new approach for 3D printed components was demonstrated called printed optics. The new method enable sensing, display, and illumination elements to be directly embedded in the casing or mechanical structure of an interactive device straight out of the 3D printer. Using these elements, new display surfaces, novel illumination techniques and special custom made optical sensors can be easily and quickly fabricated in a highly customizable manner.
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The research team used a 3D printer and a high resolution transparent plastics material with similar optical properties to plexiglas which was simple to work with and cheap. The idea for the future will be to create a full library of electronic and optical components (together with mechanical parts) from which developers could choose from. They will then create new devices in a simple and cheap manner by simply letting the 3D printer combine them together.

The Carnegie Mellon and Disney researchers demonstrated several interesting applications although many more could be envisioned for the future. For example a printed digital chess set that has the location of each piece (a good way to play with a computer with a physical chess set), volumetric displays that have pixels in 3D which can be embedded inside other devices or components. A large variety of sensors and cheap and simple controlling devices that can control and give feedback to any component from a kid's game to home automation systems.
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More information on printed optics can be found in the following paper (PDF).
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TFOT published many 3D printing related articles in recent years including "the future of 3D printing" from 2011, 3D printing on demand - a coverage of Shapeways.com beta testing 3D printing service and more recently FORM 1: hgh resolution 3D printing for the masses.