I occasionally hear the question of whether you can create a one node SQL Server cluster. That basically means you have a virtualized instance of SQL Server with a virtual IP and virtual name that are differerent from the host box, but that there is only one "possible owner" of that SQL Server. Yes, this is allowed and supported. Often it is used for testing or when you have to get the system up and running now but only have one physical server available at the moment. You can add additional nodes to the Windows cluster later, then optionally add those nodes as potential owners of the SQL Server instance. See the "add node" topics in BOL, the exact steps will vary by version. Basically "add node" means that setup adds the SQL binaries and some registry keys to the new node and updates a few cluster related registry keys so that the cluster "service" becomes aware of the additional node as a "possible owner" of that instance of SQL Server. You still only have one copy of the SQL Server databases no matter how many nodes are possible owners.
Choose "Other Versions" if you want to view the BOL topic for something other than the most recent version of SQL Server:
How to: Add or Remove Nodes in a SQL Server Failover Cluster (Setup)