Kapalua's blog: "youtube autostart"

all you have to do is get rid of everything before the first "embed" and after the second "embed" til it looks like this below. 

<embed src="http://www.youtube.com/v/s3iyAFFElb4" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed> 

after that you add to the url "&autoplay=1" 
like below 

<embed src="http://www.youtube.com/v/s3iyAFFElb4&autoplay=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed>