You need to create the djConfig object before including the dojo core (comes with ArcGIS API for JavaScript). Here's a modified version of your code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html dir="ltr">
<head>
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/2.5/js/dojo/dijit/themes/claro/claro.css"
/>
<style type="text/css">
body, html { font-family:helvetica,arial,sans-serif; font-size:90%; }
</style>
<script type="text/javascript">var djConfig = {parseOnLoad: true};</script>
<script type="text/javascript" src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=2.5"></script>
<script type="text/javascript">
dojo.require("dijit.TitlePane");
</script>
</head>
<body class=" claro ">
<div id="tp2" dojoType="dijit.TitlePane" title="I'm a TitlePane Too">
Click arrow to close me.
</div>
</body>
</html>