For complex business models, I prefer a mix of ActiveRecord and DDD. The domain objects know how to save themselves and data actions are done against a repository (nHibernate can act as a generic repository, if you look at a repository as something that exposes data to the model as a collection). The business logic resides in the domain entities, and even some encapsulation of value types can be accomplished, although only when there is a business need. Some implementations of DDD favor removing all public setters and only modifying entities through methods. I'm not a fan of that implementation unless there is a very good business need.
It seems to me that this implementation gives you the ease of use of ActiveRecord and the business logic encapsulation of DDD.
