Why on earth did you change your Nullable type model from nhibernate descriptor only (interface based converters) to a model where you utilise NHibernate specific types in your entity classes?
This is one reason why everybody got away from Ejb, so they didnt have technology specific dependencies in their application classes?
Is this just to make way for .net 2.0, and will disappear when NHibernate moves to C# 2.0?

I don't understand what you mean by "nhibernate descriptor only" - you can always write your own type, create a IUserType implementation for it and use that. No need to get so upset about something you don't have to use, I would say.

Yes, nullable types were created as a temporary solution until NHibernate supports .NET 2.0. I will probably also add types for Microsoft SqlTypes in the near future, as these are also nullable, and a bit more "standard".