Fedora ladspa plugins pakages install their .so in /usr/lib64/ladspa on x86_64, however the gstreamer ladspa plugins hardcodes the search path to /usr/lib etc and doesn't look in lib64 (Once you install ladspa plugins you also have to rebuild the gstreamer registry but that's a known GStreamer FAQ)
http://kojipkgs.fedoraproject.org/packages/gstreamer-plugins-good/0.10.11/ Please test the latest fc10 build here and report back.
Seems to fix the problem: now gst-inspect reports ladspa plugins
gstreamer-plugins-good-0.10.11-4.fc10 has been submitted as an update for Fedora 10. http://admin.fedoraproject.org/updates/gstreamer-plugins-good-0.10.11-4.fc10
gstreamer-plugins-good-0.10.11-4.fc10 has been pushed to the Fedora 10 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with su -c 'yum --enablerepo=updates-testing update gstreamer-plugins-good'. You can provide feedback for this update here: http://admin.fedoraproject.org/updates/F10/FEDORA-2009-0659
gstreamer-plugins-good-0.10.11-4.fc10 has been pushed to the Fedora 10 stable repository. If problems still persist, please make note of it in this bug report.