If possible, the attributes in NHibernate.Validator.Constraints should be in a separate DLL assembly without any dependencies (NHibernate.Validator.dll would then depend on that DLL).
My reasoning behind this is that persistence is an infrastructure concern, while validation is a business concern. So I don't want my domain assembly to have a reference to NHibernate, while a reference to NHibernate.Validator would be acceptable. But the NHibernate.Validator assembly references the NHibernate assembly, so by referencing NHibernate.Validator I get a transitive reference to NHibernate. This weakens clean separtion of concerns (unless I use XML instead of annotations, which I really do not want). Having a seperate assembly that contains only what is needed to declare validation rules without leaking persistence functionality would in my opinion cleanly solve that problem.
Unless the attribute classes themselves make use of NHibernate's persistence infrastructure, I guess this should be easy to implement.