Exchange Server 2007's Cluster Continuous Replication (CCR) feature provides a way to set-up geographically-dispersed clusters to protect against data center failure (aka "site failure"). Though the documentation provides plenty of detail on how to set up CCR clusters in a single data center - where both cluster nodes and the computer hosting the File Share Witness are in the same data center - the documentation on how to set this up across data centers has been skimpy, or even non-existent.
http://msexchangeteam.com/archive/2007/04/25/438185.aspx
See also:
http://exchangepedia.com/blog/2007/04/ccr-over-wan-failover-and-fsw-questions.html
http://exchangepedia.com/blog/2007/02/exchange-server-2003-sp1-bag-of-goodies.html