I think I remember an old thread about testing heightmaps in NWN. I think it was Primogenitor that was spearheading the effort to see if this would work.
It seems it has been implemented into NWN2 by some creative folks here:
http://nwn2yatt.sourceforge.net/
I knew this would be big if it happened. Check it out.
(Now if they could just figure out the interiors...)