What a puzzle! And I can't seem to find anything about it documented anywhere. Sort of a catch-22 right? Start the SQL instance in single-user mode, but some connection is coming in first. I suspect that you are right, that the clustering service is connecting to do its IsAlive and LooksAlive polling. Perhaps SQL Agent? Can you stop the agent service first?

The only idea I had was to change the configuration of SQL Server in Cluster Administrator first... settings on the advanced tab, to do not restart, and change the poll interval on the Looks Alive and Is Alive to a 5 minute period or something.

Darn, we are trying to build a recovery plan in case we need to restore system databases, so I am very interested in hearing how one gets connected after starting in single-user mode.

Any Clustering Gurus out there?