This template doesn't exist in VS 2010. It was built-into VS 2008, but did not require the ajax control toolkit (it isn't related to the toolkit). 
The ajax webform is identical to a regular webform except that it has a script manager and a placeholder javascript function for pageLoad. I imagine the VS team didn't think it justified having its own item template in VS 2010 (I agree).
Just select "Web Form" from the new item dialog. Then add a script manager like this:
<body>
    <form id="form1" runat="server">
        <div>
          <asp:ScriptManager ID="ScriptManager1" runat="server" />
        </div>
    </form>
</body>
You can also optionally add the script in the header if you need it... it should look like this:
<head runat="server">
    <title></title>
    <script type="text/javascript">
      function pageLoad() {
      }
    </script>
</head>