Complexity really exhibits itself in management and scale of the cluster.
Rocks does installation management and upgrading very well and we are working on improving the monitoring of a cluster. The first thing to manage is the physical deployment of a cluster. Many sites have devoted time to how to physically construct a cluster. The cluster cookbook can be a good resource. There is also an O'Reilly Book called Building Linux Clusters where almost all of it is devoted to the physical setup of a cluster, how to choose a motherboard, etc.