Postback problem when PopupExtender is placed inside a user control
<script type="text/javascript" language="javascript">
    function OkClicked(sender, e) {
            __doPostBack('Button1', e);
        }
</script>

<asp:Button ID="Button2" runat="server" Text="Show" />
<asp:Panel ID="Panel1" runat="server">
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
</asp:Panel>
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
    DropShadow="True" OkControlID="Button1" PopupControlID="Panel1"
    TargetControlID="Button2" onokscript="OkClicked()">
</asp:ModalPopupExtender>
<p>
    <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
</p>

protected void Button1_Click(object sender, EventArgs e)
{
    Label1.Text = TextBox1.Text;
}

<script type="text/javascript" language="javascript">
function OkClicked(sender, e) {
        __doPostBack('<%= this.Button1.ClientID %>', e);
    }