Steam user Rico has discovered a hidden 3D mode for PA!?
Anyone can try it out. I wonder if the devs left it there on purpose for us to discover.. or didn't finish cleaning up the code.

Go to Extras, and look at the lower left corner of the page, you'll see a little hidden highlight.. click it, and the game switches to 3D mode. You can do the same thing to go back to 2D.