Exchange 2007 SP2 has been released at last.
You can download it from here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=4c4bd2a3-5e50-42b0-8bbb-2cc9afe3216a
The service pack is so large because it is the complete installation files. You can install a new server using this download only.
Release Notes are : http://download.microsoft.com/download/8/3/E/83E9DB24-0041-4F7E-A0DD-26043BBF7CAA/RelNotes.htm
The what is new document is here: http://technet.microsoft.com/en-us/library/ee221150.aspx
This update required Windows Installer 4.5 which you can download from here: http://www.microsoft.com/downloads/details.aspx?FamilyId=5A58B56F-60B6-4412-95B9-54D056D6F9F4&displaylang=en
If you using Exchange 2007 as part of SBS 2008, then you should take note of this blog posting from the SBS Team: http://blogs.technet.com/sbs/archive/2009/07/30/microsoft-exchange-2007-sp2-installation-is-blocked-on-windows-sbs-2008.aspx
(Blog post updated to include links to release notes and what is new)