Ever wanted to write some XAML like Text="{Binding SomeValue, StringFormat={Binding SomeFormat}}"? You can't. But here's a way to get close, using attached properties.

StringFormatHelper.cs

public static class StringFormatHelper
{
	#region Value

	public static DependencyProperty ValueProperty = DependencyProperty.RegisterAttached(
		"Value", typeof(object), typeof(StringFormatHelper), new System.Windows.PropertyMetadata(null, OnValueChanged));

	private static void OnValueChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
	{
		RefreshFormattedValue(obj);
	}

	public static object GetValue(DependencyObject obj)
	{
		return obj.GetValue(ValueProperty);
	}

	public static void SetValue(DependencyObject obj, object newValue)
	{
		obj.SetValue(ValueProperty, newValue);
	}

	#endregion

	#region Format

	public static DependencyProperty FormatProperty = DependencyProperty.RegisterAttached(
		"Format", typeof(string), typeof(StringFormatHelper), new System.Windows.PropertyMetadata(null, OnFormatChanged));

	private static void OnFormatChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
	{
		RefreshFormattedValue(obj);
	}

	public static string GetFormat(DependencyObject obj)
	{
		return (string)obj.GetValue(FormatProperty);
	}

	public static void SetFormat(DependencyObject obj, string newFormat)
	{
		obj.SetValue(FormatProperty, newFormat);
	}

	#endregion

	#region FormattedValue

	public static DependencyProperty FormattedValueProperty = DependencyProperty.RegisterAttached(
		"FormattedValue", typeof(string), typeof(StringFormatHelper), new System.Windows.PropertyMetadata(null));

	public static string GetFormattedValue(DependencyObject obj)
	{
		return (string)obj.GetValue(FormattedValueProperty);
	}

	public static void SetFormattedValue(DependencyObject obj, string newFormattedValue)
	{
		obj.SetValue(FormattedValueProperty, newFormattedValue);
	}

	#endregion

	private static void RefreshFormattedValue(DependencyObject obj)
	{
		var value = GetValue(obj);
		var format = GetFormat(obj);

		if (format != null)
		{
			if (!format.StartsWith("{0:"))
			{
				format = String.Format("{{0:{0}}}", format);
			}

			SetFormattedValue(obj, String.Format(format, value));
		}
		else
		{
			SetFormattedValue(obj, value == null ? String.Empty : value.ToString());
		}
	}
}

Usage.xaml

<!-- You can't do this. -->
<TextBlock Text="{Binding Value, StringFormat={Binding FormatString}}" />

<!-- But you can do this. -->
<TextBlock 
	local:StringFormatHelper.Format="{Binding FormatString}"
	local:StringFormatHelper.Value="{Binding Value}"
	Text="{Binding (local:StringFormatHelper.FormattedValue)}"
	/>

Please be advised that this will only let you format with decimal and hex modifiers like N3 or X2. If you want to add extra text like "The average temperature is {0:N1} degrees Celsius" you have to remove the if-statement on line 71 to line 74.