I posted a patch here that uses the Validation Block. It uses the same constraints that were originally enforced by .netTiers using attributes. You can "own" or override the validation in the subclasses by specifying different or additive rules. The Validation Block will use the subclass rules over the base class. If there are more complex validation scenarios the subclass can also add any method with a [SelfValidation] attribute and a parameter of ValidationResults to perform additional validation. It would also be trivial to implement (provided my feature request is granted) a config file based validation file, which in my opinion is the most flexible solution for applications using the Validation Block.
Edit: Also one issue left unresolved. The validation block can specify different rulesets. Currently it uses "NetTiers" as the ruleset name. It would be nice to expose a property somewhere to change the ruleset at runtime. Where would be a good place to do this. I can see it being useful to set global and entity specific rulesets.