The latest version of the Exchange 2007 update is Update Rollup 5 for Exchange Server 2007 – this can be downloaded from here (64 bit and 32 bit versions now available).
Microsoft plan to do these releases rather than issue hotfixes as the method of engineering Exchange has changed since the previous versions, and KB937194 describes why this is. Each update rollup contains all the previous updates, so you only need to deploy this patch and not any earlier patches as well.
If you have not yet installed Exchange 2007 yet, copy this patch to the Update folder on your installation point and it will get slipstreamed into the installation automatically upon running Setup.
Note that unlike updates #1 and #2, a 32bit version of this update is available so this update can be applied to test and virtual computer systems and labs.