I guess you have noticed that in Debian 6 KDE the check “Enable Auto-Login” is disabled:
However you can re-run systemsettings as root and set this check on. First, make sure that you grant everyone to access X server:
volodymyr@sv1:~$ xhost +
access control disabled, clients can connect from any host
And now you can run systemsettings as root:
volodymyr@sv1:~$ sudo systemsettings
It will look now like this:
Click Apply, reboot machine and see if it works.