Google - Autoplay em vídeos embed do Youtube

Sem Autoplay:

<iframe width="420" height="315" 
src="http://www.youtube.com/embed/YPqVO2L3K7M" 
frameborder="0" allowfullscreen></iframe>

Com Autoplay:

<iframe width="420" height="315" 
src="http://www.youtube.com/embed/YPqVO2L3K7M?autoplay=1" 
frameborder="0" allowfullscreen></iframe>

Fonte: http://brianwong.com/blog/2012-youtube-embed-code-autoplay-on-and-all-other-parameters/