I highly recommend that you read the following whitepaper on achieving high availability with SQL Server 2008 if you are planning or currently designing a high availability solution for your SQL Server environment.

This whitepaper goes through the architecture one of our reference customers utilised to achieve 99.99% uptime of their SQL infrastructure.

High Availability and Disaster Recovery at ServiceU: A SQL Server 2008 Technical Case Study

http://msdn.microsoft.com/en-us/library/ee355221.aspx