a occhio, mi pare che manchi il <div id='flashcontent'> nell'html proposto

prova così

codice:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="it" lang="it">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Anteprima con FlashLinghtBox</title>
<script type="text/javascript" src="lightbox.js"></script>
<script type="text/javascript" src="flashobject.js"></script>
<style type="text/css">
@import "lightbox.css";
img { border:0; }
</style>
</head>
<body bgcolor="#ffffff">

<div id='flashcontent'>My Flash Content</div>

<script type="text/javascript">
var so = new SWFObject("intro.swf", "mymovie", "1024", "700", "6", "#FFFFFF");
so.addParam("quality", "best");
so.addParam("wmode", "transparent");
so.addParam("salign", "t");
so.write("flashcontent");
</script>

</body>
</html>