Clustering SQL Server first requires a clustered OS. And that has IMHO, been the missing element to setting up a clustered SQL Server. either limitations in windows or the IT staff just would not, or could not setup and maintain a cluster.

But lately, ease and IT skill in setting up an OS cluster have improved. That combined with mirroring being phased out will lead more and more DBA's to cluster. It makes sense to cluster, but you need the infrastructure.