Microsoft has released update rollup 6 for Exchange 2007 SP3 deployments. For a list of changes that are included in this update rollup, see KB2608656.
Exchange Server 2007 SP3 must be installed prior to installing this rollup. This update cannot be installed if you are using Exchange Server 2007 RTM, SP1 or SP2. Any interim updates (from Microsoft PSS) for Exchange Server 2007 must be uninstalled prior to installing this update rollup.
Forefront users should disable it before installing the update (fscutility /disable) and enable it afterwards (fscutility /enable).
Download the rollup here.