Not sure if this will help but...
I created a page (see code below) with your above code in and ran it through a html tidy app, then tested it at w3c.org and it validates XHTML 1.0 Strict.
As you will see it has altered a few things -
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script type="text/javascript">
//<![CDATA[
document.write('something<br />something else<br />and a little more...');
//]]>
</script>
<title></title>
</head>
<body>
</body>
</html>
let me know if this helps, or if you find out the problem would like to know what was causing it.