>Since most unix boxes and applications are very happy to have multiple >instances >of the same application in use at the same time, can you help clarify what this >sort of API accomplishes or is used for?
I think this is a rather common requirement for desktop apps. In UNIX, it would commonly be used on a per-login basis (sort of thing you'd get a DBUS session bus instance for) but remember that its also effectively used by init, and for server stuff its handy if you have a daemon that is wanting to receive all UDP traffic broadcast to a particular address.
