The markup extension that's supposed to make today's faulty, pain-in-the-ass code work.

gistfile1.cs

    /// <summary>
    ///     XAML markup extension to make it possible to reference nullable types.
    /// </summary>
    [MarkupExtensionReturnType (typeof (Type))]
    public class NullableExtension : TypeExtension
    {
        /// <summary>
        ///     Initializes a new instance of the NullableExtension class.
        /// </summary>
        public NullableExtension ()
        {}

        /// <summary>
        ///     Initializes a new instance of the NullableExtension class, initializing the TypeName value based on the provided
        ///     typeName string.
        /// </summary>
        /// <param name="typeName">
        ///     A string that identifies the type to make a reference to. This string uses the format
        ///     prefix:className. prefix is the mapping prefix for a XAML namespace, and is only required to reference types that
        ///     are not mapped to the default XAML namespace.
        /// </param>
        public NullableExtension (string typeName)
            : base (typeName)
        {}

        /// <summary>
        ///     Initializes a new instance of the NullableExtension class, declaring the type directly.
        /// </summary>
        /// <param name="type">The type to be represented by this NullableExtension.</param>
        public NullableExtension (Type type)
            : base (type)
        {}

        /// <summary>
        ///     Returns an object that should be set on the property where this extension is applied. For
        ///     <see cref="T:System.Windows.Markup.TypeExtension" /> , this is the <see cref="T:System.Type" /> value as evaluated
        ///     for the requested type name.
        /// </summary>
        /// <returns>
        ///     The <see cref="T:System.Type" /> to set on the property where the extension is applied.
        /// </returns>
        /// <param name="serviceProvider">
        ///     Object that can provide services for the markup extension. The provider is expected to
        ///     provide a service for <see cref="T:System.Windows.Markup.IXamlTypeResolver" />.
        /// </param>
        /// <exception cref="T:System.InvalidOperationException">Member value for the extension is null.</exception>
        /// <exception cref="T:System.ArgumentException">
        ///     Some part of the typeName string did not parse
        ///     properly.-or-<paramref name="serviceProvider" /> did not provide a service for
        ///     <see cref="T:System.Windows.Markup.IXamlTypeResolver" />-or- typeName value did not resolve to a
        ///     type.
        /// </exception>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="serviceProvider" /> is null</exception>
        public override object ProvideValue (IServiceProvider serviceProvider)
        {
            if (serviceProvider == null)
                throw new ArgumentNullException ("serviceProvider");

            var basis = (Type) base.ProvideValue (serviceProvider);
            return typeof (Nullable<>).MakeGenericType (basis);
        }
    }