UpdatePanel problem

[Jeff]

hey

asp.net 3.5

I'm expermenting with UpdatePanel (started learning about UpdatePanel
tonight). I've have a webpage which contain a UpdatePanel etc... In the code
behind I try to update the TextBox in the UpdatePanel, but the TextBox isn't
updated.. or well the TextBox doesn't display the value...

Here is the markup:
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server"
UpdateMode="Conditional">
<ContentTemplate>
<asp:TextBox ID="TextBox2" TextMode="MultiLine" Width="100%"
Height="50" runat="server"></asp:TextBox>
</ContentTemplate>
</asp:UpdatePanel>

Here is the behind code:
protected void OnResponse(IAsyncResult ar)
{
/* some code is removed */
TextBox2.Text = "fgsgsfgsgsgsdgsdgsdgsfdg";
UpdatePanel1.Update();
}

I've set a breakpoint in the code above, so I know it get executed

any suggestions why "fgsgsfgsgsgsdgsdgsdgsfdg" isn't displayed in the
TextBox ?