Esse link tem um ótimo white paper sobre alta disponibilidade no SQL Server 2008.  

Summary:

This white paper describes the technologies available in SQL Server 2008 that can be used as part of a high-availability strategy to protect critical data. As well as describing the technologies in detail, the white paper also discusses the various causes of downtime and data loss, and how to evaluate and balance requirements and limitations when planning a high-availability strategy involving SQL Server 2008.

  http://msdn.microsoft.com/en-us/library/ee523927.aspx