SP2 adds support for Windows Powershell v2, and allows coexistence with Exchange Server 2010. SP2 also adds support for VSS backups of Exchange 2007 on Windows Server 2008.
Exchange 2007 SP2 updates the Active Directory schema. Details of schema changes, including new attributes and classes, and modifications to existing ones can be found in Active Directory Schema Changes (SP2).

Note, once you install SP2, you cannot uninstall it without uninstalling Exchange 2007 from the server.

Microsoft recommends upgrading Exchange 2007 servers in the following order:

  1. Client Access Servers (CAS)
  2. Unified Message (UM) servers
  3. Hub Transport servers
  4. Edge Transport servers
  5. Mailbox servers

More details and important deployment considerations in Exchange 2007 SP2 Release Notes.

Links:

What’s New in Exchange Server 2007 SP2
http://technet.microsoft.com/en-us/library/ee221150(EXCHG.80).aspx

Exchange 2007 Service Pack 2 Prerequisites
http://msexchangeteam.com/archive/2009/08/28/452209.aspx

CCR upgrade:
http://www.howexchangeworks.com/2009/08/upgrading-exchange-2007-ccr-to-sp2.html