Having hardcoded text with a binding in a TextBlock

There is, if you are on .Net 3.5 SP1

<TextBlock Text="{Binding Path=Artist.Fans.Count, 
                 StringFormat='Number of Fans: {0}'}" />

In using the above approach:

<TextBlock Text="{Binding Path="Artist.Fans.Count, 
                  StringFormat='Number of Fans: {0}'}" />

I found it somewhat restrictive in that I couldn't find a way to bold face inside the StringFormat nor could I use an apostrophe in the StringFormat.

Instead I went with this approach, which worked better for me:

<TextBlock TextWrapping="Wrap">
    <Run>The value</Run>
    <Run Text="{Binding Path=MyProperty1, Mode=OneWay}" FontWeight="Bold" />
    <Run>was invalid. Please enter it with the format... </Run>
    <LineBreak/><LineBreak/>
    <Run>Here is another value in the program</Run>
    <Run Text="{Binding Path=MyProperty2, Mode=OneWay}" FontWeight="Bold" />
</TextBlock>                    

Here the binding value(clouds.all) is added with "%". You can add any value you want after "\{0\}".

 <TextBlock Text="{Binding Path=clouds.all, StringFormat=\{0\}%}"/>

Use Binding.StringFormat:

<TextBlock Text="{Binding Artist.Fans.Count, StringFormat='Number of Fans: {0}'}"/>