So, what you need is to install the gstreamer updates from updates testing with:
yum --enablerepo=updates-testing update gstreamer\*
And then make sure you also update webkitgtk to 1.2.0-1 (it should hit the repositories soon, if you can't wait grab it from koiji, the Fedora build system).
At this point you can enjoy Youtube with HTLM5/WebM encoding in GStreamer based browsers, that is epiphany and midori.
Can't wait to see WebM playback everywhere in Fedora 14!