Microsoft Knowledge Base Article 324949 contains the following summary:
In Windows 2000 and Windows Server 2003 domain controllers, the default containers for users, computers, and groups that are created by using earlier-version application programming interfaces (APIs) are of objectclass CN instead of the more desirable organizational unit class container.
Microsoft recommends that administrators aggressively deploy the best-practice organizational unit structure in all Active Directory domains (see the "More information" section for details). After you upgrade or deploy Windows Server 2003 domain controllers in Windows Server 2003 Domain mode, redirect the default containers that the earlier-version APIs use to create users, computers, and groups to an organizational unit container that the administrator specifies.
Important If you are using Microsoft Exchange Server, you must not move the Exchange Domain Servers group or the Exchange Enterprise Servers group to any other organizational units. For Exchange to function as expected, these groups must remain in the default Users container.