I was playing around with a program called StereoPhoto Maker which creates 3D picture files in .MPO format that you can view on a Nintendo 3DS, and I remembered that one of my favorite artists had created a couple of these. So I thought I'd take a crack at it using one of his recent Tinkerbell pictures.
I wrote up a quick cheat-sheet on how I did this. You can also download the PSD Photoshop file to see how I approached everything. Basically I created a depth map by hand and then used Photoshop's
on the original image to create a second image with a shifted viewpoint. The main process took about 2 hours.