DNS and active Directory Integration – E

I know AD needs DNS for name resolution finding servers etc, however, Is my DNS structure, my AD structure?
If I have a W2K/DNS domain called test.com and add a new child domain dom1, hence dom1.test.com, does the test.com and dom1.test.com domainsautomatically trust each other?
Hence all domains within the same DNS tree automatically trust each other. While domains in disjointed DNS namespaces must be joined with Explicit trusts.
Right? Wrong? Have I missed somthing?