Having a Mission-Critical Environment means a system downtime could mean a lot. Here comes the high availability comes in, and with clustering this give our service availability very high. In the upcoming Solaris 11, high availability is the main focus of Oracle. Solaris Cluster covers a lot of applications, zfs and virtualized zones.
To check out all the best that would be coming out with Solaris Cluster check out this podcast.
For more info check Solaris Cluster Website