When you run the prepare schema command (/PS) using the SETUP CLI for Exchange 2010 it fails with the following error:
Setup encountered a problem while validating the state of Active Directory. The Active Directory schema version (14625) is higher than the Setup’s version (14622). Therefore, PrepareSchema can’t be executed.
This is happens if you have installed Exchange 2007 SP3, which updates the schema to a later version than Exchange 2010 RTM.
You do not need to run the /PS command, only the /PrepareAD one or just run setup from the GUI. The GUI setup will detect the schema is updated and skip that step.
Similar blog post with issue:
Also posted as an Issue on my Issue & Tips list: