Although not particularly well-documented, you can use DevForce to validate non-entities.
With entities you get built-in validation, so with non-entities you have to drive the validation yourself. In your property setter you can call VerifierEngine.ExecuteBeforeSet and/or ExecuteAfterSet for the property in question. To perform instance-level validation you call VerifierEngine.Execute. A VerifierEngine can be created standalone, or you can use your EntityManager's. You can use standard DevForce validation attributes on non-entities, and also create custom verifiers.
Although we don't have a sample, we do make use of non-entity validation in the DevForce Silverlight Business Application template. If you've used that, the VerifiableObject class defined there serves as the base class for non-entity classes requiring verification. The VerifiableObject also implements INotifyPropertyChanged and INotifyDataErrorInfo. You can see this code in action if you create a new solution from this template, or I can upload the class if you're interested.