Using nested compositions leads to a deadlock.
Here's helgrind output:

==18146== Thread #5: Attempt to re-lock a non-recursive lock I already hold
==18146==    at 0x4C26D35: pthread_mutex_lock (hg_intercepts.c:397)
==18146==    by 0xA2291C9: gnl_composition_event_handler (gnlcomposition.c:919)
==18146==    by 0x913A903: gst_pad_send_event (gstpad.c:4634)
==18146==    by 0xA230635: no_more_pads_object_cb (gnlcomposition.c:1583)
==18146==    by 0x626B25C: g_closure_invoke (in /usr/lib/libgobject-2.0.so.0.1800.2)
==18146==    by 0x6280F5C: (within /usr/lib/libgobject-2.0.so.0.1800.2)
==18146==    by 0x6282607: g_signal_emit_valist (in /usr/lib/libgobject-2.0.so.0.1800.2)
==18146==    by 0x6282B32: g_signal_emit (in /usr/lib/libgobject-2.0.so.0.1800.2)
==18146==    by 0x9114E81: gst_element_no_more_pads (gstelement.c:835)
==18146==    by 0xA229BBC: gnl_composition_ghost_pad_set_target (gnlcomposition.c:1004)
==18146==    by 0xA23044D: no_more_pads_object_cb (gnlcomposition.c:1574)
==18146==    by 0x626B25C: g_closure_invoke (in /usr/lib/libgobject-2.0.so.0.1800.2)

The deadlock is on COMP_OBJECTS_LOCK. After some debugging i came up with
this patch:

Index: gnl/gnlcomposition.c
===================================================================
RCS file: /cvs/gstreamer/gnonlin/gnl/gnlcomposition.c,v
retrieving revision 1.80
diff -u -r1.80 gnlcomposition.c
--- gnl/gnlcomposition.c	28 Dec 2008 15:28:40 -0000	1.80
+++ gnl/gnlcomposition.c	6 Jan 2009 17:57:49 -0000
@@ -1000,8 +1000,11 @@
     gst_pad_set_active (comp->private->ghostpad, TRUE);
     if (!(gst_element_add_pad (GST_ELEMENT (comp), comp->private->ghostpad)))
       GST_WARNING ("Couldn't add the ghostpad");
-    else
+    else {
+      COMP_OBJECTS_UNLOCK (comp);
       gst_element_no_more_pads (GST_ELEMENT (comp));
+      COMP_OBJECTS_LOCK (comp);
+    }
   }
   GST_DEBUG_OBJECT (comp, "END");
 }

I _think_ it's safe to release the lock, but i'm not 100% sure.