su problems with xdm

[Alisdair McDiarmid <alisdair@letdown.demon.co.uk>]

I've got problems running X apps as root with xdm.

If I log in as alisdair (my main user) and su root in a terminal
window, any attempt to run an X application fails:

[root%letdown /home/alisdair] # xmcd
Xlib: connection to ":0.0" refused by server
Xlib: Client is not authorized to connect to Server
Error: Can't open display: :0.0

This doesn't occur if I use startx as alisdair, and then su, or
login from xdm as root. Why is this happening and how can I fix
it?

Thanks,
-- 
alisdair mcdiarmid
[i won't tear again i won't breathe in the shards of what is left]