For all of you who only have one eye (like me). The following is from an optical engineer who i am sure knows more about the specific subject:
"One of the major mechanisms for 3d vision is what is known as parallax. Parallax is the effect that objects near you move faster than objects far away when you move your head, such as when you are driving along a road the mountain in the distance seems not to move at all whereas the rock next to the road moves extremely quickly. This effect is what dominates 3d vision on distant objects (i.e. objects that appear essentially identical to the two perspectives of your eyes). When you view something that is say 30-50 feet away from you, both eyes are seeing effectively the same image but you can still tell the difference from that object and one that is 100 feet away due to parallax. Perspective also comes into play here, meaning that the farther away objects are the "smaller" they appear, and your mind has learned the generic relative sizes of certain objects (mountains are larger than trees) and that comes into play as well, which allows you to see some 3d perspective in 2d still images.
If you were to remove your stereoscopic vision (by covering one eye for example) you would remove your mind's ability to discern if it was 3d or not by the double perspective. Depending on how your individual mind has been trained over the years (if you had bad eyesight growing up and didn't have it corrected for a long time parallax would be more dominant than stereoscopic vision for example) you may experience something that appears to look like more "real" 3d than if your brain had the full information. With only 1 eye, a painting of a scene would appear to be the same as window out to the actual scene, assuming it was a distance away and your brain would ignore the focus effect as a result."