our friend Erika L. thinks this may be of some use for Andre, given his last update.

ScreenPointToRay(position: Vector3): Ray;


Returns a ray going from camera through a screen point.
Resulting ray is in world space, starting on the near plane of the camera and going through position’s x,y pixel coordinates on the screen position.z is ignored.
Screenspace is defined in pixels. The bottom-left of the screen is 0,0; the right-top is pixelWidth,pixelHeight.

// Draws a line in the scene view going through a point 200 pixels
// from the lower-left corner of the screen
function Update
var ray : Ray = camera.ScreenPointToRay Vector3200,200,0;
Debug.DrawRay ray.origin, ray.direction 10, Color.yellow;

via Unity Script Reference:.