un esempio minimale con jquery corners, preparato giusto in qualche minuto

codice:
<!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" lang="it" xml:lang="it">
<head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  <title>jQuery Corners</title>

  <script src="jquery-1.2.6.min.js" type="text/javascript"></script>
  <script src="jquery.corners.js" type="text/javascript"></script>

<script type="text/javascript">
  $(document).ready(function(){
    $('div.rounded').corners();
  });
  </script>
</head>

<body>

    <div class="rounded {12px}" style="background: #ccc; padding: 10px;">
        <span>jQuery Corners 0.3</span>
    </div>
</body>
</html>
funziona senza problemi su Firefox3, Chrome, Opera, IE6