i won't recommend using this.
anyway here is the code.
paste this above </head>
<script type='text/javascript'>
var img_thumb_width=192;
var img_thumb_height=120;
function createVideoThumb(pID,postlink){
var div = document.getElementById(pID);
if (div.innerHTML.indexOf("http://www.youtube.com/watch?v=")!=-1){
var vidid = div.innerHTML.substring(div.innerHTML.indexOf("http://www.youtube.com/watch?v=")+31,div.innerHTML.indexOf("endofvid"));
}else {var vidid =""};
if (vidid ==""){var imgvid ='';
}else{
var imgvid ='';
};
div.innerHTML =imgvid;
}
function createVideoThumb2(pID,postlink){
var div = document.getElementById(pID);
var imgtag = "";
if (div.innerHTML.indexOf("http://www.youtube.com/watch?v=")!=-1){
var vidid = div.innerHTML.substring(div.innerHTML.indexOf("http://www.youtube.com/watch?v=")+31,div.innerHTML.indexOf("endofvid"));
}else {var vidid =""};
var textinside = div.innerHTML.substring(div.innerHTML.indexOf("[starttext]")+11,div.innerHTML.indexOf("[endtext]"));
var embedvid = "";
if (vidid!="") {
embedvid = '<object width="654" height="393"><param name="movie" value="http://www.youtube.com/v/'+ vidid + '&hl=en&fs=1&rel=0&autoplay=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/'+ vidid + '&hl=en&fs=1&rel=0&autoplay=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="654" height="393"></embed></object>'
}
var summary = embedvid + '<div class="content">' + textinside + '</div>';
div.innerHTML = summary;
}
</script>
and replace <data:post.body/>
with
<div expr:id='"summary" + data:post.id'>
<data:post.body/>
</div>
<b:if cond='data:blog.pageType == "index"'>
<script type='text/javascript'>
createVideoThumb("summary<data:post.id/>","<data:post.url/>");
</script>
<b:else/>
<script type='text/javascript'>
createVideoThumb2("summary<data:post.id/>","<data:post.url/>");
</script>
</b:if>
