I've been running through your howto without any major
problems. mplayer and gmplayer are both fine from the
mplayerplug-in is fine when "use-gui" is set to "no" or
"yes". Everything works as expected. Sadly, when I set
it to "mini" nothing happens at all. It does this for
the plugger site, it does this for the DD trailer.
I've turned on debugging to see if that helps at all.
If you need any more info, let me know.