Oh wow, this is amazing. I've got to try to make sense of the tut sometime and try for myself.
Great work.
EDIT:
Tried one. ^_^
wow
Noticed a couple things whilst making it.
1. The backfilling is required because for every pixel you move the popout one way, it reverses its appearance the OTHER way (works on two planes - left/right and front/back. So you're actually making a DOUBLE sterogram, one in front and to the left, one behind and to the right [in the tut's moves]).
2. This is more easily noticable on not-very-well-tiled backgrounds like mine, but every stereogram has the image drawn onto the background (behind the popouts. See the "wow" that's stuck there? If you've got good eyes, you can see the ABC in the background of OP's sterogram. Faintly, but it's there). Any ways around this?