Major change in this Service Pack is that it enables you to install Exchange Server 2007 on Windows Server 2008 R2. It will also allow you to install the Exchange 2007 Management Tools on Windows 7.
Another nice addition is the new password reset tool for OWA users; no more fiddling around with the IISADMPWD in IIS on Client Access Servers. Enabling the password reset rool just requires creating or setting the following registry key on CAS servers (its disabled by default):
HKLM\SYSTEM\CurrentControlSet\Services\MSExchange OWA\ChangeExpiredPasswordEnabledValue (REG_DWORD) = 1
More information on the new password reset tool can be found on TechNet here.
Note that the download contains the full set of binaries; you can use it to upgrade existing installations but it can also be used to deploy new Exchange 2007 SP3 installations. Note that upgrading or installing Exchange 2007 SP3 on a system which is going to be or has been upgraded from Windows Server 2008 to Windows Server 2008 R2 in any way is unsupported.
The Exchange versions, builds and dates table has been updated accordingly and can be found here.