We are enabling slaveOK to read from secondary servers, but we have run into many problems with one of our servers that sometimes gets too far behind to be a useful reader. Therefore, the client should not query from any secondary servers that are out of date. Apparently the ability is there for the clients to query from the server, though I'm not sure if there's a way to tell the client to only read from one of the servers: http://www.mongodb.org/display/DOCS/Replica+Set+Internals#ReplicaSetInternals-ReadingfromSecondariesandStaleness