not too terribly difficult... you'll be editing the "navbar" template (about a quarter of the way down through the file). first you need to add a "name" attribute to the login form...
HTML Code:
<!-- login form -->
<form action="login.php" method="post" onsubmit="md5hash(vb_login_password,vb_login_md5password)">
changes to
HTML Code:
<!-- login form -->
<form action="login.php" method="post" name="vb_login_form" onsubmit="md5hash(vb_login_password,vb_login_md5password)">
Then just add the following javascript to the end of the login form section...
HTML Code:
<script type="text/javascript">
document.vb_login_form.vb_login_username.focus();
</script>
<!-- / login form -->