(!) [Kapil] Since Knoppix is based on Debian it is likely that it
    uses the Xwrapper program. This is controlled by
    /etc/X11/Xwrapper.config on a "sarge" version of Debian. This file
    contains the "allowed_users" option with possible values of
    "rootonly", "console" and "anybody". 
    (!) [Thomas] Kapil, you're not wrong, and indeed I actually use
    Knoppix purely as a rescue CD, preferring my version of Debian
    Sarge to be pure. Mind you, I might get around to running Knoppix
    in a chroot jail at some point.... 
    (!) [Kapil] For more details "man Xwrapper.config" is your best
    bet. Since this file can be managed by "debconf" you could also
    try to run 
    | dpkg-reconfigure -plow xserver-common |