If you are using NHibernate as your O/R Mapper, you may want to check out the recent release of NHibernate Validator for validating your business objects / entities in your domain model.
Per the NHibernate Validator Project:
"NHibernate Validator is a powerful and extensible framework to validate objects using the .Net Platform. Born as a port of Hibernate Validator 3.0.0 project at december of 2007. Later on, many improvements and changes were made to the Core and API, and now we can say that NHibernate Validator is more than just a port. NHibernate Validator is entirely written in C#. The first release of NHibernate Validator was entirely developed by Dario Quintana, Gustavo Ringel and Fabio Maulo. We appreciate the help of Davy Brion in the revision of this document."
Just like with the Enterprise Library Validation Application Block, NHibernate Validator can be use as attributes on the properties of your business objects or described via XML. You can also create custom validators for use with NHibernate Validator.
NHibernate Validator is part of the NHibernate Contrib Project and you can download it here.
NHibernate Tutorials