I'm runnig SuSE 9.2 but I've had this problem in prior versions and with Mandrake as well. In X, say KDE for instance, if I am running a console, I can't run any X apps while I'm in SU mode. I always get:

Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified

Error: Can't open display: :0


or similiar error messages. This does not happen if I am in a console as a user. Remember, I'm not talking about a text mode console. I'm talking about a console like "konsole" in X.

How can I solve this problem other than not to be in SU mode which is necessary sometimes?

the problems is because when you su you Switch User, to root, while the X display is owned by you, when root users tries to connect it cant, so thats were the error comes from ..... tho in GNOME i can use X apps when i su'd to root ... but I'm not sure what GNOME did to allow it, tho I'm positive it can be done on any non GNOME desktop as well ..

Interesting. When I sudo an app as a user, it works.