To try and troubleshoot some network manager problems I wanted to use dbus-monitor with the system bus. This bus is a lot more locked down than a user's session bus so doesn't work out of the box (at least on jaunty). Experimentation wasn't helped by the fact that the daemon seems to need a full restart to properly pick up the configuration changes.
I have been able to get dbus-monitor working by making the <policy user="root"> section in /etc/dbus-1/system.d/NetworkManager.conf contain the following:
<allow eavesdrop="true" receive_requested_reply="false"/>
<allow eavesdrop="true" send_requested_reply="false"/>
and in the <policy at_console="true"> section
<allow send_destination="org.freedesktop.NetworkManager"
eavesdrop="true"/>
<allow eavesdrop="true" send_requested_reply="false"/>
This allows dbus-monitor to pick up method calls and replies when run as root with an "at_console" user sending the method calls (e.g. with the network manager applet).