My friend and colleague, Tim McMichael, recently blogged about a behvaior change that was made in Windows Server 2008 R2 that was back-ported to Windows Server 2008. The basics of the behavior change are as follows:
When the server hosting the File Share Witness becomes unavailable, the File Share Witness resource will fail and cause the Cluster core resources to move between nodes. Assuming the File Share Witness server remains unavailable, it's corresponding cluster resource will remain in a failed state. If the Cluster service determines that the File Share Witness resource is needed to maintain quorum, and the witness resource is in a failed state, the Cluster service will attempt to bring the witness resource online. If the attempt is successful, quorum is maintained. If the online attempt is not successful, the cluster will lose quorum.
The back-port is in the form of a hotfix (KB978790). You can download the hotfix from http://support.microsoft.com/kb/978790. Note: The package is marked for the product Windows Vista, but it is actually for Windows 2008.
The hotfix is applicable to both Exchange 2007 (when running on Windows Server 2008) and Exchange 2010.