Clarification from Tutorial

During our informal tutorial, I pointed out that a position (VPOS) was indeed available to be passed in to a pixel shader.  However, it is not the vertex's position as I may have said or implied, but rather the pixel's position on the screen.  If you wanted to know where that pixel would have been in the 3D world, you would indeed need to "cheat" by passing the information in some other way (possibly using texture coordinates, as the prof was suggesting).