The User Guide states that Server 2008 does not support marking the entire database as authoritative:
"To mark AD objects, subtree, or the entire AD database as authoritative, Recovery Manager for Active Directory
uses the capabilities provided by the Ntdsutil.exe tool supplied with Microsoft Windows. However, the version of
this tool included with Windows Server 2008 does not support marking the entire AD database as authoritative.
As a result, the wizard increments the version number of the attributes of all objects in the entire directory, all
objects in the subtree, or the particular object to make it authoritative for the directory."