This works in Chrome but not Firefox 3.6 (warning: RickRoll video):
<iframe width="420" height="345" src="http://www.youtube.com/embed/oHg5SJYRHA0?autoplay=1" frameborder="0" allowfullscreen></iframe>
The JavaScript API for iframe embeds exists, but is still posted as an experimental feature.
UPDATE: The iframe API is now fully supported and “Creating YT.Player objects – Example 2” shows how to set “autoplay” in JavaScript.