My DCs OS are 2003/2003R2 and FFL:2003,DFL:2003
Wanted to create a new child domain with Windows 2008 but got various error and after creating the below  key I have promoted that 2008 DC successfully.
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Create the below reg key and after promoting the DC remove that.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Netlogon\Parameters\


Click Add Value on the Edit menu, and then add the following registry value:

Value name: NT4Emulator

Data type: REG_DWORD

Radix: Hex

Value data: 0x1

Quit Registry Editor.


See the below link; it's the same issue with 2008:
http://support.microsoft.com/kb/298713