AJAX :: Calendar Control Not Popping Up?

Recently upgraded to VS2010 and just (today) downloaded the latest version of the toolkit. The calendar doesn't pop up when my date textbox receives focus. I never encountered this problem with VS2008 and have many of these controls installed and working fine.

I have the toolkit dll in the bin directory and have added a reference to it. The project builds fine - no errors. The page code looks fine - no errors or ugly blue squiglies. In design view everything looks fine - no error messages.

The implementation in question is about as basic as it gets:

The code:

In web.config <Pages:

<controls>
<add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="ajaxToolkit" />
</controls>

On the page:

<asp:ScriptManager EnablePartialRendering="true" ID="ScriptManager1" runat="server" />
<asp:TextBox ID="StartDateTextBox" runat="server"
autocomplete="off"
Width="100px" />
<ajaxToolkit:CalendarExtender ID="CalendarExtender1" runat="server"
TargetControlID="StartDateTextBox"
Enabled="True" />