xmlroff is available prepackaged for Ubuntu 8.04! Instead of my reciting the list of packages that you need to build xmlroff, I just need to tell you to install it from the ‘universe’ repository using the Synaptics package manager.
Thanks must go to W. Martin Borgert and others of the Debian XML/SGML Group for doing the packaging work so that Ubuntu could pick it up as well as to the Ubuntu folks for including it.