Re: Some servers do not forward X11, but I do not know why.

[Jay Cribb [GMail]]

One other thing to check is the ownership of the .Xauthority file in  
the home directory of the account.
Using su to run an X program as root usually munges the file  
ownership.  Then the next time you log in, you can't get your DISPLAY  
to set since root owns the file 0600.
Simply deleting .Xauthority and then logging out and back in will fix  
it.  The file gets created if it isn't there.