Hi all,
i have updated the r.out.vtk module. It supports now different VTK
Dataformats for elevation representation. Output formats are :
* polydataset with quads (default)
* polydataset with triangle-strips
* polydata with vertices (no grid are created, it can be created with
vtkDelauny2D Filter from Paraview)
* structured grid (not recommended)
i have tested it with several datasets and it works fine, but it would be
very interesting for me, if anyone else can test it too. 
r.out.vtk is available at:
http://www-pool.math.tu-berlin.de/~soeren/grass/modules/
Best regards
soeren