Alright, I can reproduce the bug if run 'grml-x -force fluxbox'. This recreates 
the xorg.conf and fails. If I use 'grml-x -force -module vboxvideo fluxbox' it 
works fine. We ship virtualbox-ose-guest-utils with grml[-medium] 2008.11 so I 
don't understand why it shouldn't work there, though this might be an issue on 
the 64bit version. Gebi, can you please investigate on that?

regards,
-mika-