Cluster the servers for a proper solution.
http://technet.microsoft.com/en-us/library/bb124521(EXCHG.80).aspxhttp://www.simple-talk.com/sysadmin/exchange/exchange-2007-mailbox-server-clustering/This keeps the second machine up-to-date at all times and allows you to auto-failover.
Having a second box with data snap-shots on it still means a lot of work if the primary exchagne server dies.
All your users AD info will have to be changed to the second server. (not ideal)