You can deploy Windows Server 2008 Read Only Domain Controllers (RODC)in a Windows Server 2003 forest.

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Quizplus · Accepted Answer

Windows Server 2008 RODCs can be deployed in a Windows Server 2003 forest. However, it is important to note that some features that are available in Windows Server 2008 R2, such as the ability to cache passwords and manage credential caching, are not available when deploying an RODC in a Windows Server 2003 forest.

You Can Deploy Windows Server 2008 Read Only Domain Controllers