I had a thinko and when trying to add a non-voting RS member, used the form
rs.add("host:port, votes: 0")

Here's an excerpt from the logs:

Fri Sep 9 13:04:43 [conn25285] Uncaught exception, terminating
Fri Sep 9 13:04:43 dbexit:
Fri Sep 9 13:04:43 [conn25285] shutdown: going to close listening sockets...
Fri Sep 9 13:04:43 [conn25285] closing listening socket: 5
Fri Sep 9 13:04:43 [conn25285] closing listening socket: 6
Fri Sep 9 13:04:43 [conn25285] closing listening socket: 7
Fri Sep 9 13:04:43 [conn25285] closing listening socket: 8
Fri Sep 9 13:04:43 [conn25285] removing socket file: /tmp/mongodb-27017.sock
Fri Sep 9 13:04:43 [conn25270] got request after shutdown()
Fri Sep 9 13:04:43 [conn25161] got request after shutdown()