Microsoft have announced the release of Update Rollup 4 for Exchange Server 2007 SP1. You can download the update here.
The update file has “v2” in it due to the accidental release of an earlier build of this rollup back in September. This rollup will install over v1 if you happened to install that one.
Be sure to read the KB article to understand the impact on Client Access Servers and also Outlook Web Access customisations.