To address questions on extending schema:
Release notes (http://download.microsoft.com/download/8/3/E/83E9DB24-0041-4F7E-A0DD-26043BBF7CAA/RelNotes.htm) point to the article on how to prepare AD for Exchange 2007 (also updated for E2007 SP2): http://technet.microsoft.com/en-us/library/bb125224.aspx
Just to make sure, I just ran through this in my lab: downloaded publicly available SP2 bits. Installed Windows Installer 4.5. Then ran setup /ps from the folder where SP2 files were extracted to. It ran and did extend the schema.
Are we saying that this does not work for some people?
C:E2007SP2RTM>setup /ps
Welcome to Microsoft Exchange Server 2007 Unattended Setup
Preparing Exchange Setup
No server roles will be installed
Performing Microsoft Exchange Server Prerequisite Check
Organization Checks ......................... COMPLETED
Configuring Microsoft Exchange Server
Extending Active Directory schema
Progress ......................... COMPLETED
The Microsoft Exchange Server setup operation completed successfully.