Service Pack 3 for Exchange 2007 is now available.
Aside from including all the updates up until Exchange 2007 SP2 Update Rollup 4, Service Pack 3 also allows you to install Exchange 2007 on a Windows Server 2008 R2 server.
Download: Exchange 2007 Service Pack 3
View: Release Notes for Exchange Server 2007 SP3
View: What’s New in Exchange Server 2007 SP3