Disaster recovery protection for Microsoft Exchange Server - Computerworld:

Microsoft Exchange server downtime costs companies millions of dollars a year. Technically savvy IT organizations are therefore working to eliminate or lessen the impact of both planned and unplanned downtime through the implementation of high-availability systems and Disaster-recovery systems. However, building a geographically dispersed Exchange cluster is not a simple task; it requires proper planning, testing, deployment and validation. By understanding the issues raised in this discussion and considering these recommendations during the planning stage, you will have taken the first and most critical step toward deploying the system that best meets your business objectives.

Having worked (not on exchange, fortunately) at an organization , where the exchange server was often down (like, 2 or 3 times a week, often for about half a day), I can honestly say that I appreciate how vital a fully functional communications system is. Even worse, when the 'chosen' system isn't considered reliable, people tend to revert back to more analogue methods - like written memos, or post-it notes - blech.