Binding only part of a label

The Binding.StringFormat property doesn't work on Labels, you need to use the ContentStringFormat property on the Label.
For example, the following sample will work:

<Label>
    <Label.Content>
        <Binding Path="QuestionnaireName"/>
    </Label.Content>
    <Label.ContentStringFormat>
        Thank you for taking the {0} questionnaire
    </Label.ContentStringFormat>
</Label> 

The same as short Version:

<Label Content="{Binding QuestionnaireName}" ContentStringFormat="Thank you for taking the {0} questionnaire" />

Using it to display a unit after the value:

<Label Content="{Binding Temperature}" ContentStringFormat="{}{0}°C" />

While this sample will not:

<Label>
    <Label.Content>
        <Binding Path="QuestionnaireName" StringFormat="Thank you for taking the {0} questionnaire"/>
    </Label.Content>            
</Label>

If you're using 3.5 SP1, you can use the StringFormat property on the binding:

<Label Content="{Binding Order.ID, StringFormat=Order ID \{0\}}"/>

Otherwise, use a converter:

<local:StringFormatConverter x:Key="StringFormatter" StringFormat="Order ID {0}" />
<Label Content="{Binding Order.ID, Converter=StringFormatter}"/>

With StringFormatConverter being an IValueConverter:

[ValueConversion(typeof(object), typeof(string))]
public class StringFormatConverter : IValueConverter
{
    public string StringFormat { get; set; }

    public object Convert(object value, Type targetType,
                          object parameter, CultureInfo culture) {
         if (string.IsNullOrEmpty(StringFormat)) return "";
         return string.Format(StringFormat, value);
    }


    public object ConvertBack(object value, Type targetType,
                              object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }

That'll do the trick.

[Edit : Change the Text property to Content]