Relative file names put on the command line for the
Windows (XP SP2) version of Inkscape are taken to be
relative to the Inkscape installation directory rather
than the current directory.
The name of the file to be opened has to be fully
qualified otherwise it's not found. If the name of the
file for the --export-png option does not have any
directory specification then the exported file is put
in the Inkscape directory (e.g., C:\Program
Files\Inkscape 0.43) rather than the current directory.
This is a right pain if you want to create a make file
which is portable across multiple systems.
This problem doesn't seem to apply on Linux (Ubuntu)
where the current directory is used correctly.