Apple has patented a 3D display system that requires neither special glasses or parallax screens, and which will supposedly enable "inexpensive auto-stereoscopic 3D displays that allow the observer complete and unencumbered freedom of movement." The system would instead use a combination of eye-tracking and a special, reflective display that would monitor the position of the user and bounce the image from a projector so as to split 3D content for the left and right eye.
Apple describes the special display involved as "a projection screen having a predetermined angularly-responsive reflective surface function," which basically means that the angle of light reflection from different points on the screen would be predicable enough for a computer to bounce light with individual eye accuracy. It's unclear whether Apple's system would be able to support more than one simultaneous viewer, or indeed what computational requirements such a setup might demand.
The application was filed back in 2006, and of course there's no guarantee that Apple ever intends to produce 3D-capable hardware using the technology it covers.