teoricamente basterebbe solo quello si! solo che a me non funziona...bah
ti riporto l'intera pagina :
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Dialog.aspx.cs" Inherits="Dialog" %>
<!DOCTYPE html>
<html>
<head>
<title>LogIn</title>

<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="http://code.jquery.com/mobile/1.1.1/jquery.mobile-1.1.1.min.css" />
<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.1.min.js"></script>
<script type="text/javascript" src="http://code.jquery.com/mobile/1.1.1/jquery.mobile-1.1.1.min.js"></script>

</head>
<body>
<div data-role="page">

<div data-role="header" data-theme="b">
<h1>Cook App</h1>
</div>

<div data-role="content" data-theme="c" >
Go!
</div>

</div>

<div data-role="page" id="diag">

<div data-role="page" id="Diag_New">
<div data-role="header" data-theme="b">
<h3>sei sicuro?</h3>
</div>
<div data-role="content">


hhhh</p>
</div>
</div>
</div>
</body>
</html>