Tenía la intención de deshabilitar y habilitar los botones fuera del TabControl, al igual que aquellos dentro del TabItem cuando se cambia la pestaña actual. Pero los enlaces de comando de TabItem no parecen impactar "hacia arriba" el árbol visual. Quées la forma correcta de hacer esto?

Con este XAML:

<Window x:Class="WpfApplication10.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication10"
    Title="MainWindow" Height="350" Width="525">
<StackPanel>
    <Button Content="MyCommand1" Command="local:MainWindow.MyCommand1" />
    <Button Content="MyCommand2" Command="local:MainWindow.MyCommand2" />
    <TabControl>
        <TabItem Header="tabItem1" Name="tabItem1">
            <TabItem.CommandBindings>
                <CommandBinding Command="local:MainWindow.MyCommand1" 
                                Executed="ExecuteMyCommand" />
            </TabItem.CommandBindings>
            <StackPanel>
                <Button Content="MyCommand1" Command="local:MainWindow.MyCommand1" />
                <Button Content="MyCommand2" Command="local:MainWindow.MyCommand2" />
            </StackPanel>
        </TabItem>
        <TabItem Header="tabItem2" Name="tabItem2">
            <TabItem.CommandBindings>
                <CommandBinding Command="local:MainWindow.MyCommand2" 
                                Executed="ExecuteMyCommand"/>
            </TabItem.CommandBindings>
            <StackPanel>
                <Button Content="MyCommand1" Command="local:MainWindow.MyCommand1" />
                <Button Content="MyCommand2" Command="local:MainWindow.MyCommand2" />
            </StackPanel>
        </TabItem>
    </TabControl>
</StackPanel>
</Window>

Con este código detrás:

    public static readonly RoutedUICommand MyCommand1 = new RoutedUICommand();
    public static readonly RoutedUICommand MyCommand2 = new RoutedUICommand();
    public MainWindow()
    {
        InitializeComponent();
    }
    private void ExecuteMyCommand(object sender, ExecutedRoutedEventArgs e)
    {
        MessageBox.Show("Hello");
    }