The Windows Vista Team Blog have an article on enabling Parental Controls on a Domain-Joined Machine

The question came up as to whether Parental Controls would remain in effect if you were to join your computer to a domain -- for instance, the corporate network here at MS (or maybe you run a domain controller at home). If you're a system administrator, you probably already know that Parental Controls is disabled when you join your Windows Vista PC to a domain. This is because it's a consumer feature not generally designed for use in domain-joined scenarios. Since many of us here at MS use our production laptops for demonstrations as well, we were previously required to hack the registry in order to re-enable Parental Controls after joining the domain. Otherwise, a demonstration of Parental Controls was impossible. But now that Group Policy is broadly enabled on the Windows Vista builds we're using, a simple modification to the Management Console will change it back.

Enabling Parental Controls on a Domain-Joined Machine