In this blog post Tuna Toksoz shows us how to use Fluent Validation with his db4o facility. He implements this validation using the events Creating and Updating which are raised before updated/added objects are stored back in db4o (Thanks Tuna!)
(Another shot at validation and db4o is in this blog post. In this case Dario Quintana integrates NHibernate.Validator with db4o)