The one thing you are forgetting about is the SQL Server unless you already have that clustered. If the SQL Server goes you are screwed.

We've been looking at this for awhile but wanted an automated solution so we are to use 2 products from Neverfail Group (http://www.neverfailgroup.com); Neverfail for SQL Server and Neverfail for BlackBerry (the Domino version is not release yet but we are working with them on the product).