To make sure its a DRY approach all validation logic should of course go in the business logic (model).
- How are validation messages presented to views, should be able to localize error messages
- Can you generate javascript from the validation framework. Compatibility with JQuery would be perfect
- Is the framework compatible with a DbC approach?
Edit: I think this is the nicest one until now, Castle validator + live validation http://blog.codeville.net/2008/04/30/model-based-client-side-validation-for-aspnet-mvc/