When people first started networking computers together so they worked as one they were actually groups of workstations (Old Sun/CGI workstations).
These were refered to as a NOW (Network of Workstations).
When the University of Wisconsion (sp? bah) They named thier's COW.
Needing to come up with a C word, they used Cluster to make COW be a Cluster of Workstations.
And thats where the term Cluster got its start to be used as a group of computers.
Mooo i tell you, Mooooo