A Quick Note on Shaders

A vertex shader function must always return an initialized value tagged as POSITION0, but then, the pixel shader function can't touch it. It throws an obscure error, along the lines of invalid input semantic 'POSITION0'

Learned that the hard way :/

If you really need to access the position, you need to copy it into a variable tagged TEXCOORD0, and then use that. It makes sense, really, if you think about it... Positions are for vertex shaders, textures are for the pixel shaders! Ish, oh well.