I'd like to propose systemd (GPL2+, http://www.freedesktop.org/wiki/Software/systemd) as blessed external dependency for GNOME 3.2.
Currently the interfacing between GNOME and systemd is minimal. Bastien has been implementing a UI for changing the host name via a configuration UI in the control center which uses a tiny mechanism daemon included in systemd as backend. GLib already exposes g_get_user_runtime_dir() which is a frontend for XDG_RUNTIME_DIR whose only implementation I know right now is in systemd.
In the future I expect more interfacing with GNOME however, and I'd thus like to see the discussion regarding acceptance as blessed dependency started early.
In the long run I expect the following additional interfaces used by GNOME or one of its components: ...