If you have not heard the announcement elsewhere, Enterprise Library 4.0 from patterns & practices is now available.  Martin Bennedik just updated his WPF integration for the Validation Application Block (VAB) to work with Enterprise Library 4.0.  He has also added some very convenient features.  This is a MUST HAVE if you are looking to use VAB for validation of your business objects/entities.  The ErrorProvider from Martin makes surfacing the errors in your WPF UI so easy!