What are you planning to do with unconnected nodes ?
If you just wanna remove them, there's "Delete Unused Nodes" option in the Hypergraph window, under Edit menu.
If you perhaps need those nodes for something, you could easily write script for that - create a loop that checks
every node in scene for connections (in MEL, "listConnections" command), and if there is any connection on current node, skip that node;
if there is no connection, add current node to selection (or add to string array, etc); and continue until all nodes are checked.
Or something similar.
Take care,
Darko