I am aware that slackware wont let you connect to X server as root for security reasons.
eg # kwrite /etc/X11/xorg.conf
will output the error 'cannot connect to X server'.

But my question is this; I am am using a terminal as root and then i issue
# su - david
and i still cannot use a command like the eg given above, the error 'cannot connect to X server' appears again. Is there a way around this? At the moment i open another tab in Konsole and this works, but would like to know the correct way to su.

type su then enter then password. Or kdesu and the program you want to run kde su kwrite. now if you do this a couple times and then you cant use kwrite as super user then log out of the user then log in as root then log back out it will remove the stale lock file for the ICE.auth
if you have a full install and are not using 3 party nvidia drivers. type xorgsetup then after your done setting it up type kdm. then sign in as a user.

My problem is not really with xorg.conf, or any file in particular. If I su from user -> root -> user, i cannot connect to X server as user any more. Can this be fixed somehow?

If you include a minus sign when 'su'ing, the environment is switched to that of the new user. The problem with running X programs is that the $DISPLAY environment variable is lost. If you make note of what its value is before switching (it is typically ":0.0") then you can reset it after switching users (e.g., "DISPLAY=:0.0 kwrite").

When using 'su', you could omit the minus sign and thus retain the original user's environment. Alternately, when switching to root using 'su', the "-s" switch will cause retention of the $DISPLAY value (and the current working directory) but other environment variables (such as $PATH) will be updated.

try it. or alt + F2 then type kdesu and the name of the program you want to run as root.
this has been a nice security feature for the kde4 desktop and the kdesu is set up to work in xffce4 also.
the original ones that other distro's use are su- and that does not work in Slackware.
just keeping the back door closed. So you use the kdesu program to run as root.

Thanks, I will remember in future to use kdesu.

Would the $DISPLAY variable be reset if I were to create a .bashrc file? At the moment my installation of Slackware doesn't use one.

read this this has been around a long time