Slithe's Journal: What is preventing FLOSS Nvidia Drivers?

What information is Nvidia not providing that would allow one to write FLOSS drivers for Nvidia cards? I am not an expert on this topic, so this is why I am asking Slashdot. A modern graphics card is basically a piece of hardware that can do matrix and vector operations very quickly, and modern graphics cards are programmable with shaders. I know that Nvidia graphics cards use the GPU assembly language, whose instructions are available and form the basis of writing shaders. I also know that X.Org has an open-source Nvidia driver for basic (i.e. not 2d or 3d accelerated) operation of the graphics card. Could someone take the Mesa OpenGL library and write a shader or set of shaders for every OpenGL function that the card should accelerate? What am I not seeing that would allow this to work?