Compatibilidad con SQL 2008 HierarchyID en NHibernate

Le he dado una prueba a la respuesta de Needles. Es una muy buena respuesta, pero se necesitan algunos cambios para que funcione (al menos en .NET 4). Esto es lo que se me ocurrió para mi proyecto:

Actualización: El siguiente código se puede descargar en GitHub y se actualizará allí. NHiberntate.HierarchyId.UserType

SqlHierarchyId IUserType

namespace NHibernate.UserTypes
{
    using SqlTypes;
    using System;
    using System.Data;
    using System.Data.SqlTypes;
    using Microsoft.SqlServer.Types;

    public class HierarchyId : IUserType
    {
        #region Properties

        public SqlType[] SqlTypes
        {
            get { return new[] { NHibernateUtil.String.SqlType }; }
        }

        public Type ReturnedType
        {
            get { return typeof(SqlHierarchyId); }
        }

        public bool IsMutable
        {
            get { return true; }
        }

        #endregion Properties

        #region Methods

        new public bool Equals(object x, object y)
        {
            if (ReferenceEquals(x, y)) return true;
            if (x == null || y == null) return false;

            return x.Equals(y);
        }

        public int GetHashCode(object x)
        {
            return x.GetHashCode();
        }

        public object NullSafeGet(IDataReader rs, string[] names, object owner)
        {
            object prop1 = NHibernateUtil.String.NullSafeGet(rs, names[0]);

            if (prop1 == null) return null;

            return SqlHierarchyId.Parse(new SqlString(prop1.ToString()));
        }

        public void NullSafeSet(IDbCommand cmd, object value, int index)
        {
            if (value == null)
                ((IDataParameter)cmd.Parameters[index]).Value = DBNull.Value;

            else if (value is SqlHierarchyId)
                ((IDataParameter)cmd.Parameters[index]).Value = ((SqlHierarchyId)value).ToString();
        }

        public object DeepCopy(object value)
        {
            if (value == null) return null;

            return SqlHierarchyId.Parse(((SqlHierarchyId)value).ToString());
        }

        public object Replace(object original, object target, object owner)
        {
            return DeepCopy(original);
        }

        public object Assemble(object cached, object owner)
        {
            return DeepCopy(cached);
        }

        public object Disassemble(object value)
        {
            return DeepCopy(value);
        }

        #endregion Methods
    }
}

Mapeo

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="DataLayer" namespace="NHibernate.Map">
    <class name="NHibernate.Map.OrganizationUnit, DataLayer" table="`orgunit`">

        <property name="HierarchyId" column="`ou_hid`" type="NHibernate.UserTypes.HierarchyId, DataLayer" />
        ...

    </class>
</hibernate-mapping>

Objeto con HierarchyId

namespace NHibernate.Map
{
    using Microsoft.SqlServer.Types;

    public class OrganizationUnit
    {
        #region Fields

        private SqlHierarchyId _hierarchyId;
        ...

        #endregion Fields

        #region Properties

        public virtual SqlHierarchyId HierarchyId
        {
            get { return _hierarchyId; }
            set { _hierarchyId = value; }
        }
        ...

        #endregion Properties
    }
}