I've implemented a high availability solution for a customer that has two BES hosts running concurrently - one in production and one at a DR site.

The primary SQL 2005 server is in the production data centre and is mirrored to the DR site using transaction log shipping.

If you lose the production data centre you have everything available in a 'hot' spare configuration. You just run a SQL script on the backup database to change it to being the primary database and 'move' the users from the offline BES to the online BES.

This last process takes about 20 minutes to successfully move 100 users. The online BES just pushes new service books to the user devices and everybody gets on with the job...except if your messaging system isn't covered by your DR strategy...