Good post for incorporating Nullables into current (1.0.2) version of NHibernate at
http://frazzleddad.blogspot.com/2005/10/more-on-nhibernate-and-nullables_20.html
James Avery at http://dotavery.com/blog/archive/2005/09/30/5202.aspx has graciously offered a workaround to the current lack of support to .Net2.0 Nullables.
Be sure to stick the current build of NHibernate into the VS solution James offers and then rebuild before sticking the Nullables2.dll into your projects. Otherwise, NHibernate won't resolve the type correctly.