Mouse Coordinate Transformation

Hello all, i know this topic has been discussed but i have a little twist to throw in…

I want to basicaly throw out the Y coordinate… i want the X and Z world coordinates of X and Y screen point. Ive tried and swapping the Y and Z world coords do not do the trick =)

Im looking for an effect similar to the one used in the Trainz Surveyor program at … the Y coordinate is based on the heightmap at the current mouse position (trivial)… The problem is the transformation of the z coordinate… Does anyone have any insight on this, i know i am to use glUnproject ive used it in the past, but for this effect im assuming there are quite a few more steps or calculations.