BES will run fine in a clustered Environment, but you cannot use Windows clusting to automatically fail over the BES, you must do that manually. I'm hoping 4.0 has better support for this.
What I do is I have a hot spare backup BES, which if our production BES goes does I only need to log on and fire up the services.
Its not automatic, but it takes a whole 30 seconds, no mail is lost to BB's during the downtime, and I can do it all remotely from my BB
