It's called openKMQ / openOverUnder and uses a cheap prism viewer (available since 1983) that allows for full color stereoscopic viewing of printed matters as well as LCDs.
The iPad is the first tablet class device that seemlessly works in portrait mode and is therefor a perfect match for the over/under viewing method.
The catch with the viewer is that one has to keep it at a precise distance to have a 3-D experience. This sweet spot distance is proportional to how far the pictures of right and left views are apart. The bigger the image pair, the bigger the optimum viewing distance.
A minimum size would be a postcard, so sorry this won't work with an iPhone, but you can alter the proportions so it fits any remote controlable Digital Picture Frame.
For the iPad, using the full screen (half/half), the optimum distance is 440 mm.