Bill Baer put together some fantastic guidance on how to consider database mirroring as a backup for your MOSS environment as well as what the different levels of mirroring introduce into a DR solution.  The most important part of the article are the steps that must be taken to fail over to a mirror from SharePoint.  Using stsadmin in all tiers (Web Applications, Central Admin, etc.) you have to remove and add the content database pointing at the mirror server.

http://blogs.technet.com/wbaer/archive/2007/04/23/sql-server-2005-database-mirroring-and-windows-sharepoint-services-3-0-microsoft-office-sharepoint-server-2007-part-1-introduction-overview-and-basics.aspx