While exploring client/dbclient_rs.cpp (see http://jira.mongodb.org/browse/SERVER-2377), I discovered that, apparently, the ReplicaSetMonitorWatcher runs only once per replica set, with a 20-second delay after first access to the replica set.
Maybe this is fine, but above the class declaration there is a comment, stating "global background job responsible for checking every X amount of time", which sounds like it should be a periodic task instead of a one-time background job.
I think either the watcher should run every X seconds, or the comment should be changed to state that this is a one-time per-replica set process.