What is DBUS used for?
On the (GNOME) desktop: Avahi, CUPS, GConf, Nautilus, Network Manager, Power Manager, Screen Saver, Volume Manager
System level elements: HAL, Network manager
Gnome applications: Pidgin, Empathy/Telepathy, Liferea, Rhythmbox
Other applications: skype
But also on the Openmoko platform (freesmartphone.org)