I just discovered there is a bug in the RC version of VS 2010 it is not generating the designer code for controls within an Ajax Tab Control. Have a look here

In VS 2008 the following

&lt:asp:TabContainer ID=”TabContainer1″ runat=”server” ActiveTabIndex=”1″>
&lt:asp:TabPanel runat=”server” HeaderText=”Contacts” ID=”TabPanel1″>
&lt:ContentTemplate>
&lt:asp:UpdatePanel ID=”UpdatePanel1″ runat=”server” ChildrenAsTriggers=”false” UpdateMode=”Conditional”>
&lt:ContentTemplate>
&lt:asp:TextBox ID=”txJoe” runat=”server”>
&lt:/asp:TextBox>
&lt:/ContentTemplate>
&lt:/asp:UpdatePanel>
&lt:/ContentTemplate>
&lt:/asp:TabPanel>
&lt:/asp:TabContainer>

Will generate the following line in the designer code file
             protected global::System.Web.UI.WebControls.TextBox txJoe;

However in VS2010 this does not get generated and so you cannot access the control in your code behind file. Hopefully we will get a fix soon and don’t have to wait for the RTM.