Before you can introduce Windows Server 2008 domain controllers into existing Windows 2000 or Windows Server 2003 domains, you must prepare the forest and domains with the ADPREP utility. ADPREP.exe is a command-line tool that extends the Active Directory schema, and updates permissions as necessary to prepare a forest and domain for a domain controller that runs the Windows Server 2008 operating system.
http://www.petri.co.il/windows-server-2008-adprep.htm
Some Additional stuff,
Microsoft Windows 2000 & 2003 Domain to Windows 2008 Upgrade Procedure
& Things to Know in General About Server 2008
http://www.commodore.ca/windows/server_2008/windows_server_2008_upgrade.htm
Upgrade Domain Controllers: Microsoft Support Quick Start for Adding Windows Server 2008 or Windows Server 2008 R2 Domain Controllers to Existing Domains
http://technet.microsoft.com/en-us/library/upgrade-domain-controllers-to-windows-server-2008-r2(WS.10).aspx
Guide for Upgrading to Windows Server 2008
http://technet.microsoft.com/en-us/library/cc755199(WS.10).aspx
