Try doing:
xhost + local: or xhost +
Please don't do that!
Well, ssh creates the ~/.Xauthority file, you could try setting the XAUTHORITY variable after su-ing to root. So: XAUTHORITY=~USER/.Xauthority export XAUTHORITY
that's not needed either.
Here may be something that will help. On my suse box, I can use x11 forwarding without any problem at all. When the connection is slow, I see a small xterm flash with the title 'xdg su' just before the X application starts. Maybe that will jog a memory or two.
Since I have both OS's to compare -- which files are involved in handling the authentication for x forwarding? I'll do a side-by-side comparison and report back if I can figure out which files to compare -- thanks.
Check that X11Forwarding is set to yes on the ssh *server* also check "man sshd" there's a discussion if you have a ~/.ssh/rc or /etc/ssh/sshrc executable file -- damjan