An interesting post about where we can configure the FSW (File Share Witness)on a Geographically dsipersed CCR cluster was blogged in the MsExchangeTeam.com site.
Exchange Server 2007 introduced Continuous Cluster Replication (CCR), which utilizes the log shipping and replay features of Exchange 2007 combined with a Majority Node Set (MNS) cluster. CCR utilizes the Microsoft Cluster service to provide failover to the passive node. An Exchange 2007 CCR cluster can be configured with two nodes and a File Share Witness (FSW). FSW is an option that is recommended so that a third piece of hardware does not need be utilized. FSW takes the place of the voter node that was used prior to update 921181. This hotfix allows the use of FSW which is used as a quorum resource without using a shared disk. The FSW is used by the cluster node for arbitration. At least two of the three (when counting nodes or FSW) must be up and communicating to maintain a majority or any cluster resources and the cluster service will cleanly shutdown.
You can read more at: http://msexchangeteam.com/archive/2007/04/25/438185.aspx