I just wanted to share a simple code that I wrote:
which uses Visit's VTK writer (written in C, wrapped in Python). I
wrote a pcolor() function, that works just like the one in matplotlib,
only it produces a vtk file (resp. data that you then pass to the vtk
writer). See the pictures and demos on the webpage above.
With this, it's very easy to write VTK files, no need to fiddle with
the ASCII format --- btw, the module can produces a binary VTK file
too, so it really saved me lots of troubles.