Hello
I am trying this on blender 2.5 revision 26641
blender -b /home/philippe/point-density.blend -o /tmp/test -F PNG -x -f 10
inside the blend file this another format defined. ( AVI JPEG)

the blender still save the rendered frame on the avi format

This a problem for because this broke also the net render.

i compiled this revision on ubuntu 9.10 with the following command line

./scons/scons.py -j3 WITH_BF_RAYOPTIMIZATION=1 WITH_BF_OPENMP=1 WITH_BF_FFTW3=1 WITH_BF_FFMPEG=1 WITH_BF_OPENEXR=1

I also provide the blender file used.

If more information is need tell me.

Regards
Philippe.