It's not obvious how to rotate the text in a WPF TabControl TabItem's Header. However, it is easy. :)

The trick is to adjust the TabItem's HeaderTemplate. I created a simple DataTemplate that rotated the contained content using a LayoutTransform (a RenderTransform would have clipped the content).

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Name="Window"
    Title="Window1"
    Width="640" Height="480" >

    <Window.Resources>

        <DataTemplate x:Key="TabItemHeaderTemplate">

            <Grid>

                <ContentPresenter Content="{TemplateBinding Content}" 

                                  RenderTransformOrigin="0.5,0.5" >

                    <ContentPresenter.LayoutTransform>

                        <TransformGroup>

                            <RotateTransform Angle="-90"/>

                        </TransformGroup>

                    </ContentPresenter.LayoutTransform>

                </ContentPresenter>

            </Grid>

        </DataTemplate>

    </Window.Resources>

    <Grid x:Name="LayoutRoot">

        <TabControl Margin="163,47,64,50" 

                    IsSynchronizedWithCurrentItem="True" 

                    TabStripPlacement="Left">

            <TabItem Header="A big one" 

                     HeaderTemplate="{DynamicResource TabItemHeaderTemplate}">

                <Grid>

                    <TextBlock HorizontalAlignment="Center" 

                               VerticalAlignment="Center" 

                               FontSize="128" 

                               Text="1" 

                               TextWrapping="Wrap"/>

                </Grid>

            </TabItem>

            <TabItem Header="A big two" 

                     HeaderTemplate="{DynamicResource TabItemHeaderTemplate}">

                <Grid>

                    <TextBlock FontSize="128" 

                               Text="2" 

                               TextWrapping="Wrap" 

                               Margin="143.02,83.375,143.02,83.375"/>

                </Grid>

            </TabItem>

        </TabControl>

    </Grid>

</Window>