To have a Youtube video appear behind your toolbar, you should add the parameter wmode=transparent to the end of the URL.
So change this:
<iframe width="425" height="349"
src="http://www.youtube.com/embed/H1Opn4DS88k"
frameborder="0" allowfullscreen></iframe>
to this:
<iframe width="425" height="349"
src="http://www.youtube.com/embed/H1Opn4DS88k?wmode=transparent"
frameborder="0" allowfullscreen></iframe>
More here:
If you don't want to hardcode the HTML into your posts, you should use a plugin like the one provided by @Tubal Martin. This has the advantage that if Youtube update their embed code in the future, you should only need to upgrade the plugin to the latest version.