Microsoft have released a paper which explains the process for upgrading Active Directory domains to Windows Server 2008 and Windows Server 2008 R2, how to upgrade the operating system of domain controllers, and how to add domain controllers that run Windows Server 2008 or Windows Server 2008 R2 to an existing domain.
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=fa629de2-f4dd-47ac-8d80-3db46b2877a2