The Microsoft Exchange Team has announced the release of Update Rollup 5 for Exchange Server 2007 SP1. The update is available now at the Download Center and will make its way to Microsoft Update in the next 2-3 weeks.
The support article describing the hot fixes included in the rollup can be found here, and includes this timely fix:
949893 You cannot create a new mailbox or enable a mailbox in an Exchange Server 2007 environment on February 29, 2008
Be sure to read the guidance regarding special considerations for customized OWA files and CAS-CAS deployments.