dbus-user-session is a dependency of the desktop meta packages, so uninstalling it means that you on Unity uninstall ubuntu-session and ubuntu-desktop, on GNOME Flashback you uninstall gnome-session-flashback etc.
dbus-user-session is a dependency of the desktop meta packages, so uninstalling it means that you on Unity uninstall ubuntu-session and ubuntu-desktop, on GNOME Flashback you uninstall gnome-session- flashback etc.