Without --auth:

connecting [with auth] to: localhost:27888/admin
testReplSet:SECONDARY> use local
switched to db local
testReplSet:SECONDARY> db.y.save(

{"whee":1}

)
testReplSet:SECONDARY> db.y.find()

{ "_id" : ObjectId("4ebced0bf3ff493b2fe7c468"), "whee" : 1 }

With --auth:

connecting [with auth] to: localhost:27888/flomeromity
testReplSet:SECONDARY> use local
switched to db local
testReplSet:SECONDARY> db.z.save(

{"whee":1}

)
unauthorized
testReplSet:SECONDARY> db.z.find()
error: {
"$err" : "unauthorized db:local lock type:-1 client:127.0.0.1",
"code" : 10057
}