If you need set KDE/kdm (the KDE window manager) to allow a default, no-password logon, but need to set it up from the command line, check your kdmrc file.
On Ubuntu/Kubuntu its /etc/kde4/kdm/kdmrc
These fields are important:
NoPassEnable = true
NoPassUsers = username
Obviously, this can be a big security hole. Use with care and know what you are doing.
See also: Allow X to connect to KDE
