We recently published TechNet topics about Adprep.exe
for Windows Server 2012. Adprep has many changes for this release:

• Adprep runs automatically as needed as part of
  AD DS installation. For example, if you are installing AD DS on a server
  that runs Windows Server 2012 in a forest where all existing domain controllers
  run a previous version of Windows Server, then adprep /forestprep and adprep
  /domainprep run automatically before the AD DS installation on the Windows
  Server 2012 server begins. If you are installing a read-only domain controller in this
  case, then adprep /rodcprep runs automatically in addition to the other
  commands.
• Adprep runs remotely, and targets the
  appropriate operations master roles (FSMOs) as needed. These FSMO role holders
  must be online and reachable from the server where you run Adprep.
• Although it runs automatically during AD DS
  installation, you can also run Adprep.exe separately in advance of an AD DS
  installation, as with previous releases of Windows Server and Active Directory.
• Adprep must  be run on a 64-bit server that runs Windows
  Server 2008 or later. There is no longer a 32-bit version (Adprep32.exe).
• The forest functional level must be Windows
  Server 2003 or higher in order to run Adprep.   
• Adprep /domainprep /gpprep does not run
  automatically, so it must be run separately as needed. You will still be able
  to install AD DS without running this command, but RSOP Planning Mode will
  function correctly only after this command has been run. See KB 2737129.

The following TechNet topics related
to Adprep have been updated for Windows Server 2012:

• Windows Server
  2012: Changes to Adprep.exe (this is only a navigation page)
• Windows Server
  2012: Forest-Wide Updates
• Windows Server
  2012: Domain-Wide Updates
• Windows Server
  2012: Read-Only Domain Controller Updates (there are none btw, but I included
  this topic in any case for completeness)
• Windows Server
  2012: Schema Updates (this is just a copy of the .ldf files that ship
  with the OS disk)

• Running
  Adprep.exe

• Adprep

Justin [Active Directory
Documentation Team]