See:
http://blogs.testbit.eu/timj/2008/02/05/05022008-thread-safe-class-initializers/ (page moved)
I finally got around to fix a long-standing and tricky bug report: Bug 64764 - Class initialization isn't thread safe.
Thread safety problems in class initializers and _get_type() functions caused nasty problems in other components that depend on parallel type creation, in particular GStreamer (Dependency Graph for bug 64764). With both being fixed now, testing feedback about GType/GObject threading problems using GLib trunk is appreciated.