Microsoft pushes Clustering as the next best thing to sliced bread. In my experience we have had more downtime with clusters than with a regular server. Clusters are very touchy and the smallest things can make them fail over. 100% CPU usage for a certain length of time can cause them to fail over... We have two active/passive clusters here and three active/active clusters here. Installing SQL 2008 in a clustered environment is a royal pain. I had so many problems just getting it installed I had to call support.