After using the EntLib3 validation application block (VAB) for a while now, and also some of the new configuration mechanisms such as external configuration source and environmental overrides for different build types; it occurs to me that there is a bit to much XML noise in the config.
EntLib3 should do as in Castle Windsor and MonoRail and Ruby on Rails; apply "convention over configuration". In EntLib3 config files, you have to add mandatory attributes to identify the default configuration when there are multiple config options. Using "first is default" as in the Windsor 'constructor injection' mechanism would make configuration simpler and less error prone (see VAB issue below). I understand the reasons for having a default-identifier as a separate attribute, but with the new config override and merging mechanisms in EntLib3, there should be less need for compulsory config "switching" attributes.
The convention "first is default" would prevent silly omission errors such as not setting the default rule set in VAB less drastic. As it is now, if you forget to set the VAB default rule set, no validation will be applied, neither will there be any "no rules" exception - and your code will run as if all validations passed, even if there are plenty of broken rules in the input.
While I'm at it, the caching of connection strings and service URLs in the Settings class and the service proxies, is also really silly; you will not be able to detect that some configuration is missing until you move the solution to an isolated staging environment that has no access to the databases/service resources referenced in the development environment. Most test environments are not that isolated from the development environment, and such config errors can go undetected for a long time during testing. This is one area where it would be better if Microsoft could make configuration compulsory.