Only the deployment of 2008 Active Directory Domain Controllers would require a Schema Update. Any other updates would be for additional products (e.g. Exchange, OCS, etc) if purchased.
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS
There are several PKI-based features (such as version 3 certificate templates, online responders, Network Device Enrollment Service, and Qualified Certificates) that depend on Windows Server 2008 schema extensions.
There are also some aspects of DFSR functionality that are not available without them (as per http://blogs.technet.com/askds/archive/2008/07/02/what-are-the-schema-extension-requirements-for-running-windows-server-2008-dfsr.aspx)
all domain services require the schema update and also for getting all of them running, you have to raise the functional levels to Windows server 2008. Some of the new features like "Fine grained password and lockout policy" for example.
Here you can see most important changes within 2008, not only about schema changes:
Best regards Meinolf Weber Disclaimer: This posting is provided "AS IS" with no warranties, and confers no rights.