Notes on preparing a Windows Active Directory forest for Windows Server 2008:

http://technet2.microsoft.com/windowsserver2008/en/library/1d502209-cdb9-4e13-9a6c-57ad6c9d1e8b1033.mspx
http://technet2.microsoft.com/WindowsServer2008/en/library/83cd65ef-1923-448b-8f58-4c922cae04e71033.mspx

For previously installed W2K or W2K3 Active Directory Forests:
ADPREP /Forestprep on the forest's schema master
ADPREP /rodcprep on the domain master in each domain - for introducing 2008 Read-Only DCs
ADPREP /domainprep /gpprep on the infrastructure master in each domain

Note that Microsoft recommends creating a local copy of \sources\adprep from the installation media, and running adprep.exe from there.

Which DCs hold your FSMO roles?  Run the following command on a DC...

     netdom query fsmo