Ok this is fixed now with the latest gnome-system-tools. In the "General" tab of System/Administration/Network you now have an checkbox to enable or disable avahi.
** Changed in: avahi (Ubuntu) Status: Confirmed => Fix Released -- avahi not usable for normal users https://launchpad.net/bugs/53911 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs