I installed today Ubuntu Server Edition 7.04 on one of my machines and here practically nothing is installed. Therefore for example X11 forwarding doesn’t work if you remotely log in with “ssh -X” (after you installed openssh-server :). So, if you start a X11 program you get e.g.:
xterm Xt error: Can’t open display:
xterm: DISPLAY is not set
I found the solution here. You need to install the program xauth with
sudo apt-get install xauth
and off you go.