Microsoft Knowledge Base Article 910204 contains the following summary:

This article discusses a problem with promoting a Microsoft Windows Server 2003 domain controller or a Microsoft Windows 2000 domain controller to a global catalog server. This article discusses the following topics:

The event messages that are logged in the Directory Services log for Windows 2000 Server and Windows Server 2003.
The possible causes of the global catalog promotion failure.
Ways to determine the cause of the global catalog promotion failure.
Ways to resolve the global catalog promotion failure.