Well optical illusions do work to a certain point, but not well enough to do anything more complex then what I uploaded earlier. Ok well maybe slightly more complex, but the point is it would take far too much time too make something really successful and that could actually work nicely with the grid style that minecraft uses to place blocks.
Maybe in the future we can see more Optical Illusions, but for now its just too confusing and complicated to get everything to work.
Just an idea for the future Notch
-BabyEagleFoot