D-Bus handles hotplug and application events.
D-Bus (Desktop Bus) is a IPC (Inter Process Communication) layer responsible to exchange data between applications. D-Bus has an interface to the hardware. In the past it was HAL, but since HAL is deprecated packages from the Devicekit providing a D-Bus interface are used.
There is a dbus deamon so start it:
/etc/init.d/dbus start and to have it automatically done at next boot:
rc-update add dbus default
There is a dbus useflag that are set by default.
Last but not least add the user to the plugdev group as via command line:
gpasswd -a<username>
plugdev
To get an idea what is going on type:
dbus-monitor --system
Now plug in some USB devices and observe the events.
There is also a dbus viewer qdbusviewer. If dbus does not want to run, try to start the daemon manually dbus-daemon --system. It might fail but tells you to delete /var/run/dbus-pid
and this might be the reason, a PC crash might have left it over.
D-Bus replaced bonobo (component model) used in earlier gnome environments.