Configured as per http://www.mongodb.org/display/DOCS/Master+Master+Replication

Startup commands:
/opt/mongodb-1.5.3/bin/mongod --port 27017 --dbpath /opt/tmp/mongodbdata --master --slave --source localhost:27018 --rest
/opt/mongodb-1.5.3/bin/mongod --port 27018 --dbpath /opt/tmp/mongodbdata2 --master --slave --source localhost:27017 --rest

But, neither node are master:
$/opt/mongodb-1.5.3/bin/mongo localhost:27017 --eval "printjson(db.\ $cmd.findOne(

{ismaster:1}

));"
MongoDB shell version: 1.5.3
connecting to: localhost:27017/test

{ "ismaster" : 0, "msg" : "not paired", "ok" : true }

$/opt/mongodb-1.5.3/bin/mongo localhost:27018 --eval "printjson(db.\ $cmd.findOne(

{ismaster:1}

));"
MongoDB shell version: 1.5.3
connecting to: localhost:27018/test

{ "ismaster" : 0, "msg" : "not paired", "ok" : true }

Michael Dirolf confirmed:
Okay I can reproduce this. Seems to me that ismaster should be true
when running w/ both --master and --slave. Mind opening a jira for
this?