I notice that building the gtk-docs in gst-plugins-base causes a hang, with the 
following error message:

  pkgbuild: creating gst-plugins-base-libs-scan
  pkgbuild: gtk-doc: Running scanner gst-plugins-base-libs-scan

After hacking the gtkdoc-scangobj (which builds the gst-plugins-base-libs-scan program) so that it doesn't unlink/delete the gtk-scangobj and running it by hand we also notice the hang.  A stacktrace of the process (included at the bottom of this report) shows that the problem is with the ext/gnomevfs plugin.
The existance of gst_gnomevfs_get_supported_uris highlights that the
problem is with the probe of this plugin.

Looking at the gst_gnomevfs_get_supported_uris function where the hang seems to be happening, I see it is doing the following:

  GnomeVFSURI *uri;
  gchar *uris[] = {
    "http://localhost/bla",
    "file:///bla",
    "smb://localhost/bla",
    "ftp://localhost/bla",
    "sftp://localhost/bla",
    "nfs://localhost/bla",
    "ssh://localhost/bla",
    NULL
  }
  , **result;
  gint n, r = 0;

  result = g_new (gchar *, 9);
  for (n = 0; uris[n] != NULL; n++) {
    uri = gnome_vfs_uri_new (uris[n]);
    if (uri != NULL) {
      gchar *protocol = g_strdup (uris[n]);
      gint n;

      gnome_vfs_uri_unref (uri);
      for (n = 0; protocol[n] != '\0'; n++) {
        if (protocol[n] == ':') {
          protocol[n] = '\0';
          break;
        }
      }

      result[r++] = protocol;
    }
  }

Looking at the above code, it looks like what the GStreamer VFS 
plugin is doing is passing in some URI's that are fake (localhost/bla 
probably does not point to a real file, etc.) and calling 
gnome_vfs_uri_new to see if GNOME-VFS can manage the URI.

Looking at the stack trace, the fact fact that load_module and
vfs_module_init is in the stack implies that one of them is causing a
gnome-vfs module to get loaded.  It seems that the init function for
this VFS module is wanting to contact GConf for some reason.  In our
build environment we normally do not have GConf running so this just
hangs.  I'm not sure which URI is specifically causing the problem.
I notice that commenting out all uri's from the array except for
the "file://" one makes the problem go away (probably at the expense
of disabling all other URI support from the plugin).

Here is the stacktrace of gst-plugins-base-scan at the time of the
hang, promised earlier:

 feb2ada4 read     (8, ffbfe2b3, 1)
 fe5d1b98 try_to_contact_server (1, ffbfe61c, ffffffff, 1628, 22b48, 1400) + 24
 fe5d1d1c gconf_get_config_server (0, ffbfe61c, 1628, 22a60, fe5b6635, 1400) + c0
 fe5cd8f8 gconf_engine_connect (5cfa0, 1, ffbfe61c, 5cfa0, 26e64, ffbfe5a1) + a0
 fe5cdb38 gconf_engine_get_database (5cfa0, 1, ffbfe61c, feb233f4, fecb4430, fecb42c0) + c
 fe5ce6bc gconf_engine_notify_add (5cfa0, fe447e80, fe5d5230, 35d58, ffbfe61c, 1) + 168
 fe5d57c4 gconf_client_add_dir (35d58, fe447e80, 1, ffbfe684, fffe0b70, 0) + ec
 fe43ad04 proxy_init (14, 1030, fe459f04, 1000, 1f270, 0) + 74
 fe43a04c vfs_module_init (5ad68, 0, fe459f04, 10000, 1fecc, 10000) + 18
 fe626888 load_module (5cf30, 5ad68, 0, ffbfe824, ffbfe820, 0) + 128
 fe626bb0 load_module_in_path_list (3f0a8, 5ad68, 0, ffbfe824, ffbfe820, ffbfe81c) + 58
 fe626d18 gnome_vfs_add_module_to_hash_table (0, 177e, bbc, 40c9c, fe667898, fe669b88) + 120
 fe626e9c gnome_vfs_transform_get (5ad68, 1b84, 1800, 726dc, fecb2608, 3c) + 68
 fe637258 gnome_vfs_uri_new_private (fe677dac, 0, 0, 1, fe677dac, 24) + e0
 fe6767a4 gst_gnomevfs_get_supported_uris (5cf00, ffbfe954, fe677dac, 7f4, 116f8, 5cf00) + 68
 fe674a3c gst_gnome_vfs_src_uri_get_protocols (0, fe688588, fe687e38, 750, 1342c, 400) + 34
 ff1b37e4 gst_element_register (37320, 0, 80, 3f990, 5b480, 48a40) + 290
 fe6723b4 plugin_init (37320, fe688488, fe6722f4, fe687e38, 15b40, 0) + c0
 ff1d2ee0 gst_plugin_register_func (37320, 0, fe688488, 640c0, ff236d10, ff2100e4) + 294
 ff1d362c gst_plugin_load_file (ff23a0d8, 0, 5a4e0, 3f538, 37320, fecb42cc) + 51c
 ff1d92c8 gst_registry_scan_path_level (38c20, 17000, 7, 0, 484b0, 3f538) + 580
 ff1d8ea4 gst_registry_scan_path_level (38c20, 17000, 8, 0, 476d8, 4a600) + 15c
 ff1d8ea4 gst_registry_scan_path_level (38c20, 17000, 9, 1, 484d0, 40518) + 15c
 ff1d8ea4 gst_registry_scan_path_level (38c20, 17000, a, 1, 39688, 3fdb8) + 15c
 ff1d9488 gst_registry_scan_path (38c20, 46768, 17400, ff210bba, ff236d10, ff21f650) + 134
 ff198b90 init_post (174, ff239df4, 39638, 0, 3d138, 38c20) + 52c
 fec475d8 g_option_context_parse (28f08, 0, 0, ffbff2ec, 6d854, fecb2608) + 7d4
 ff197fb4 gst_init_check (0, 0, ffbff2ec, 2c00, 9ee30, 0) + d8
 ff198004 gst_init (0, 0, feb79040, 9ed2c, ff236d10, ff082000) + 24
 00011ef0 main     (1, ffbff3bc, ffbff3c4, 27c00, ff080140, ff080180) + 28
 000114e0 _start   (0, 0, 0, 0, 0, 0) + 108