@aku: DRY is nice, but its not always possible. Validation is one of those places, as you will have three completely different and unrelated places where validation is not only possible but absolutely needed: Within the UI, within the business logic, and within the database.
Think of a web application. You want to reduce trips to the server, so you include javascript validation of client data entry. But you can't trust what the user enters, so you must perform validation within your business logic before touching the database. And the database must have its own validation in order to prevent data corruption.
There's no clean way to unify these three different types of validation within a single component.
There are some attempts being made to unify cross-cutting responsibilities like validation within policy injectors like the P&P group's Policy Injection Application Block combined with their Validation Application Block, but these are still code based. If you have validation that's not in code, you still have to maintain parallel logic separately...
