> rs.initiate()
{
        "info2" : "no configuration explicitly specified -- making one",
        "me" : "redbeard:27017",
        "info" : "Config now saved locally.  Should come online in about a minute.",
        "ok" : 1
}
foo:PRIMARY> conf = rs.conf()
{
        "_id" : "foo",
        "version" : 1,
        "members" : [
                {
                        "_id" : 0,
                        "host" : "redbeard:27017"
                }
        ]
}
foo:PRIMARY> conf.settings = {getLastErrorDefaults: {w:0}}
{ "getLastErrorDefaults" : { "w" : 0 } }
foo:PRIMARY> conf
{
        "_id" : "foo",
        "version" : 1,
        "members" : [
                {
                        "_id" : 0,
                        "host" : "redbeard:27017"
                }
        ],
        "settings" : {
                "getLastErrorDefaults" : {
                        "w" : 0
                }
        }
}
foo:PRIMARY> rs.reconfig(conf)
{ "ok" : 1 }
foo:PRIMARY> rs.conf()
{
        "_id" : "foo",
        "version" : 2,
        "members" : [
                {
                        "_id" : 0,
                        "host" : "redbeard:27017"
                }
        ],
        "settings" : {
                "getLastErrorDefaults" : {
                        "w" : 0
                }
        }
}