Hello.
I have the following ASPX page:
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title></title>
    <script src="js/jquery-1.2.6.min.js" type="text/javascript"></script>
    <script src="js/jquery-ui-1.6.custom.min.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(function() {
            $("#dialog").dialog({
                bgiframe: true,
                autoOpen: false,
                height: 300,
                modal: true,
                buttons: {
                    'Ok': function() {
                        $(this).dialog('close');
                        __doPostBack('TreeNew', '');
                    },
                    Cancel: function() {
                        $(this).dialog('close');
                    }
                },
                close: function() {
                    ;
                }
            });
        });
        function ShowDialog() {
            $('#dialog').dialog('open');
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="TreeNew" runat="server" Text="Nuevo" OnClientClick="ShowDialog(); return false;"/>
        <asp:Label ID="Message" runat="server"></asp:Label>
        <div id="dialog" title="Create new user">
            <p id="validateTips">All form fields are required.</p>
            <asp:RadioButtonList ID="ContentTypeList" runat="server">
                <asp:ListItem Value="1">Text</asp:ListItem>
                <asp:ListItem Value="2">Image</asp:ListItem>
                <asp:ListItem Value="3">Audio</asp:ListItem>
                <asp:ListItem Value="4">Video</asp:ListItem>
        </asp:RadioButtonList>
        </div>
    </div>
    </form>
</body>
</html>
When the user click on TreeNew button appears the modal dialog, then he/she choose an option an click Ok button to do a postback.
I need that the server side execute TreeNew_Click method: How can I do that?
If I use __doPostBack('TreeNew', '') it throws me the following error: "Object expected".
UPDATE:
I found the origin for the error: the function __doPostBack is not defined. I'm not going to delete the question because I think Chris Clark's answer is so interesting.