When running the gnome desktop without gdm e.g. by startx things does not work as expected. E.g. if a usb disk is plugged in it is not mounted and the err "not authorized" is displayed.
I think the correct solution may be to include ck-launch-session in startx but I have not been able do do this. I still dont really understand exactly how the ubuntu startup scripts works.
EDIT: please note that this does not fix usb automount in xbmc.
The problem can instead be fixed by:
sudo apt-get install policykit-1 devicekit-power
sudo nano /var/lib/polkit-1/localauthority/50-local.d/custom-actions.pkla
[Actions for xbmc user]