This does work!

By exchanging the right and left views it is possible to obtain (with some strain) a 3-D effect without any equipment. To view the crossed-eye view shown here, move slightly back from your normal viewing distance and place your viewpoint on a line perpendicular to the center of the image. Place your finger halfway between your eyes and the image and view your finger. The three bright spots between the pictures should become four spots, and the two images become three. If the focus of the eyes is now allowed to drift to the surface of the screen without uncrossing the eyes a three dimensional depth illusion will appear in the central image. The finger may now be removed from the view. A viewer may find that the extra side images disappear once in-depth view of the central image is stable. This is a popular way of presenting images on computers but it is difficult to learn and for many viewers the method is not comfortable enough for extended viewing. It also offers none of the advantages enumerated above that are provided by the stereoscope.

[copied directly from the Wikipedia article on Stereoscopy]