Given a plane and a 3D point, transform the 3D point to a 2D coordinate along the plane.

Excuse me if this is irrelevant, but if you have the vector towards the plane (?), perhaps this code for finding the distance from the point to the plane would be of use? I don't have a clue about 3D maths stuff though...

http://www1.acm.org/pubs/tog/GraphicsGems/gemsiii/pt2plane.c