Here is the code for the calendar extender:

<tr>
     <td align="right" height="35" style="WIDTH: 215px">Date Received:</td>
     <td style="WIDTH: 397px; HEIGHT: 32px" align="left"><asp:textbox id="txtDateReceived" runat="server"></asp:textbox>
       
                        <cc2:CalendarExtender ID="txtDateReceived_CalendarExtender" runat="server"
                          cssclass="calendarContainer"  Enabled="True" TargetControlID="txtDateReceived">
                        </cc2:CalendarExtender>
       
                        &nbsp;<asp:requiredfieldvalidator id="rfvRecdDate" runat="server"  Font-Size="X-Small" ControlToValidate="txtDateReceived"
       ErrorMessage="Enter date."></asp:requiredfieldvalidator>
      <asp:comparevalidator id="Comparevalidator2" runat="server" Font-Size="X-Small"
       ControlToValidate="txtDateReceived" ErrorMessage="Use MM/DD/YYYY format." Display="Dynamic" Operator="DataTypeCheck"
       Type="Date"></asp:comparevalidator></td>
    </tr>

If I remove the extender, I can add controls and they are recognized.  And it's not just the calendar extender, it's any Ajax extender.