Over at the Objet blog, Sam Green has a video that highlights a simple example of this concept with customized iPhone covers. On the back face of each cover there are a series of interlinked gears that when pushed will move in concert with one another.
While your projects may have more complex and dynamic working parts, have no fear. With the proper tolerances (usually 0.01” between individual features) even the most complex assemblies can be printed as fully functional prototypes.
Watch the Video at Objet