X problem as root

[Marco Calviani <mirtux@studenti.fisica.unifi.it>]

Hi everybody,
 i've got a problem whenever i become root and try to open a window in 
my user X session:

for example:

user@local:~$ su
root@local:~$ eog
Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified


Does anybody can diagnose the problem?

Thanks,
MC