Having just promoted a Windows Server 2008 system to Domain Controller status, there were some things that needed to happen. Similar to previous Windows Server releases there needed to be some updating to the schema and domain since the Active Directory was Windows 2003.
Since this new 2008 server was in a different site than the FSMO holder, the adprep.exe couldn't be run from the system that was to be promoted, I had to go to the correct system.
Once I found the correct DC (which held all the FSMO roles), adprep had to be run with up to four different switches: /forestprep (must be run on the Schema Master), /domainprep (must be run on the Infrastructure Master), /domainprep /gpoprep (again on the Infrastructure Master to update Group Policy functionality) and optionally /rodcprep (if you want to use Read-Only Domain Controllers which are very cool, run this on the Infrastructure Master as well).
Luckily nothing of note happened and all was well...it was then Guinness time!
More information can be found on the Microsoft TechNet site.