<UserControl xmlns:controls=”clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls”�
    x:Class=”SilverlightApplication18.MainPage”
    xmlns=”http://schemas.microsoft.com/winfx/2006/xaml/presentation“
    xmlns:x=”http://schemas.microsoft.com/winfx/2006/xaml“
    xmlns:toolkit=”clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Layout.Toolkit”           �
    Width=”400″ Height=”300″>
    <Grid x:Name=”LayoutRoot” Background=”White”>
        <controls:TabControl Margin=”10″ TabStripPlacement=”Left”>
            <controls:TabItem  >
                <controls:TabItem.Header>
                    <toolkit:LayoutTransformer >
                        <TextBlock Text=”Tab1″/>
                        <toolkit:LayoutTransformer.LayoutTransform>
                            <RotateTransform Angle=”-90″></RotateTransform>
                        </toolkit:LayoutTransformer.LayoutTransform>
                    </toolkit:LayoutTransformer>
                </controls:TabItem.Header>
                <TextBlock  Margin=”10″ Text=”some content in Tab1″/>              �
            </controls:TabItem>
            <controls:TabItem   >
                <controls:TabItem.Header>
                    <toolkit:LayoutTransformer >
                        <TextBlock Text=”Tab2″/>
                        <toolkit:LayoutTransformer.LayoutTransform>
                            <RotateTransform Angle=”-90″></RotateTransform>
                        </toolkit:LayoutTransformer.LayoutTransform>
                    </toolkit:LayoutTransformer>
                </controls:TabItem.Header>
                <TextBlock Margin=”10″ Text=”some content in Tab2″/>              �
            </controls:TabItem>
        </controls:TabControl>
    </Grid>
</UserControl>

From: http://leeontech.wordpress.com/2009/04/06/tabcontrol-and-vertical-text/