Comment 16 for bug 328575

I think the issue is that XAuth and D-Bus auth are two different systems so where su'ing to another user will allow an app to run as that user on the primary user's display, D-Bus will prevent the app from connecting to the primary user's session bus.

There are edge cases, however small, where the user wants to run an app using another user's configuration (for theming, permissions, remote access, etc.) but want to run on their display. Since configuration in GNOME apps are starting to require D-Bus applications can fail to start up in this configuration. G-Conf uses D-Bus to see if there are any other G-Conf instances in this session.

This is most likely a flaw in G-Conf but does in a round about way point to the issue of sessionless D-Bus requests, and what to do about them. Right now we orphan autostarted busses.