I've got set of vertices, and would like to calculate their UV coordinates by projecting them by a given normal. Similar to what "planar mapping" does in 3ds max. 

Another way of looking at it is that I want to flatten the vertex positions from a given normal direction, so that I can simply use their X & Y (or X & Z depending on your preference) coordinates as UV coordinates.

I'm thinking it's a simple matter of a mathematical Vector function, basically rotating vertex positions along the normal, but I can't figure out the math behind it. I'm using C# and XNA, so I've got Vector3, Matrix and Quaternions. Preferably as cheap as possible.

Any help?