How about this:
<Window x:Class="Transform.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Label Content="A" Height="48" HorizontalAlignment="Left" Margin="134,124,0,0" Name="label1" VerticalAlignment="Top" Width="41" FontWeight="Bold" FontSize="32">
<Label.RenderTransform>
<RotateTransform Angle="-45" CenterX="-50" CenterY="-50" />
</Label.RenderTransform>
</Label>
</Grid>
</Window>
This XAML is all that is needed. I have put the letter into a label, but this would also work with any other text and even with other controls.
Edit: You can't use transformations on a letter or a string directly, but you can use the transformation on the controls containing them.