To edit a single image with the filename "foo.svg":  

    inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileQuit foo.svg`

To edit all svg images in the current directory:

    inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileClose *.svg

But this second command will open a ton of windows, which is just bad news all around. Here's a better command (for Linux, at least):

    for img in $(ls *.svg) ; do inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileClose $img ; done