UPDATE: I couldn’t actually get Guacamole to work consistently and the connection kept dropping. I ended up uninstalling it and using a package called xrdp, which allows to remote connect to Ubuntu from a Windows machine using built-in Remote Desktop, see http://www.liberiangeek.net/2012/05/connect-to-ubuntu-12-04-precise-pangolin-via-windows-remote-desktop/ for details.
Home site at http://guac-dev.org/ and installation instructions for Ubuntu at http://guac-dev.org/Debian%20Install%20Instructions
What the installation instructions are missing is that you actually need to install a VNC tool. Couldn’t work vnc4server but x11vnc worked first time (http://www.karlrunge.com/x11vnc/) AFTER I set it up to use a password (to match the password set for for guacamole under /etc/guacamole/user-mapping.xml) using the process described here (http://www.karlrunge.com/x11vnc/faq.html#faq-passwd).