Microsoft KB http://support.microsoft.com/kb/937052 tracks all Service Packs and Rollup Updates for Exchange Server 2007. It's a useful resource to find out what is the latest update available.
This article describes how to obtain the latest update rollup for Microsoft Exchange Server 2007. Update rollups are a common way to distribute Exchange 2007 fixes and Exchange 2007 modifications. You can install the latest update rollup to help keep the product up-to-date.