"videotestsrc ! theoraenc ! ffdec_theora ! xvimagesink" fails.
"videotestsrc ! theoraenc ! rtptheorapay ! rtptheoradepay ! ffdec_theora ! xvimagesink" fails too.

This tries to use the internal theora decoder of ffmpeg (not libtheora). 

Thats probably a problem with gst-ffmpeg... Anyway, I suggest just disabling the ffmpeg implementation and going with the libtheora implementation for now.

Proposed patch:

diff --git a/ext/ffmpeg/gstffmpegdec.c b/ext/ffmpeg/gstffmpegdec.c
index 6eb0903..35c2330 100644
--- a/ext/ffmpeg/gstffmpegdec.c
+++ b/ext/ffmpeg/gstffmpegdec.c
@@ -2747,6 +2747,7 @@ gst_ffmpegdec_register (GstPlugin * plugin)
     /* MPEG1VIDEO : the mpeg2video decoder is preferred */
     /* MP1 : Use MP3 for decoding */
     /* MP2 : Use MP3 for decoding */
+    /* Theora: Use libtheora based theoradec */
     if (!strcmp (in_plugin->name, "gif") ||
         !strcmp (in_plugin->name, "vorbis") ||
         !strcmp (in_plugin->name, "mpeg1video") ||
@@ -2754,6 +2755,7 @@ gst_ffmpegdec_register (GstPlugin * plugin)
         !strcmp (in_plugin->name, "mp1") ||
         !strcmp (in_plugin->name, "mp2") ||
         !strcmp (in_plugin->name, "libfaad") ||
+        !strcmp (in_plugin->name, "theora") ||
         !strcmp (in_plugin->name, "mpeg4aac")) {
       GST_LOG ("Ignoring decoder %s", in_plugin->name);
       goto next;