Here's what pops up:
The first set of code showed up 20 times (literal, not figurative) between running the prompt to start rhythmbox and its crash. I plugged in the iPhone before rhythmbox started running.
(rhythmbox:6617): Gtk-CRITICAL **: gtk_style_provider_get_style_property: assertion `g_type_is_a (gtk_widget_path_get_object_type (path), pspec->owner_type)' failed
(rhythmbox:6617): GLib-GObject-CRITICAL **: g_value_set_object: assertion `G_VALUE_HOLDS_OBJECT (value)' failed