After I posted about how to regain tab navigation in a popup, Ian commented that the workaround had some flaws.  First, he lost intellisense inside the <UserControl> and second, which is more painful, he could not refer to any controls inside the child <UserControl> from the page's code-behind.

Well, I found a more suitable workaround.  Instead of using a <UserControl> to set the TabNagation, you can use a <ContentControl> instead.  Here's the updated workaround code:

   1: <UserControl x:Class="PopupTabbing.Page"
   2:   xmlns="" 
   3:   xmlns:x="" 
   4:   Width="400" Height="300">
   5:   <Grid x:Name="LayoutRoot" Background="White">
   6:   <Popup IsOpen="True">
   7:   <Popup.Child>
   8:   <ContentControl TabNavigation="Cycle">
   9:   <StackPanel Width="150">
  10:   <TextBlock Text="Username" />
  11:   <TextBox x:Name="Username" />
  12:   <TextBlock Text="Password" />
  13:   <PasswordBox x:Name="Password" PasswordChanged="Password_PasswordChanged" />
  14:   <Button Width="75" Content="Login" Click="Button_Click" />
  15:   </StackPanel>
  16:   </ContentControl>
  17:   </Popup.Child>
  18:   </Popup>
  19:   </Grid>
  20: </UserControl>

Thanks to Ian for pointing out the problems with the workaround!