Dear developers,
i've added z-direction-scaling support for the r3.out.vtk module and
made some simple pictures with paraview and the exported slovakian 3D GRASS
dataset. New version and pictures are availible at:
http://www-pool.math.tu-berlin.de/~soeren/grass/modules/
enjoy 
(Note: the dataset is twentyfold scaled in z-direction!)
I have also a question to ask:
"Is there a module wich creates cut-planes (as a new grass rastermap) from 3d raster files?"
If not i will try to program such a module - if it is possible, please correct me if i'm wrong. I also planing to write a module to export normal rastefiles with
elevation and stuff to the vtk dataformat. So we are able to visualize the combination
of raster, image and raster3d files in VTK (vis5D+ is to old to support and not
in development anymore 
, and VTK alo supports any vectortype you can imagine ).
Best regards
Soeren Gebbert