codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<style type="text/css">
<!--

div#bloccosx {
	border:#000 solid 3px;
float:left;
width:200px;
}
.test {
border: 1px solid #000000;
padding: 5px;
width: 150px;
height: 150px;
margin: 10px auto 10px auto;
}


-->
</style>
</head>

<body>
<div id="bloccosx">
  <div class="test">Inserire qui il contenuto per  class "test"</div>
</div>
</body>
</html>
Io lo vedo centrato sia con IE sia con FF