If you start mongod up as a shardsvr and specify the replSet without any ports, it should assume the default shardsvr port 27018, but it does not.

mongodb-linux-x86_64-1.6.1/bin/mongod --dbpath /root/data --fork --logpath log.log --shardsvr --replSet set2/ip-10-194-11-207.ec2.internal
> use admin
switched to db admin
> rs.initiate()
{
"startupStatus" : 4,
"info" : "set1/ip-10-245-118-48.ec2.internal",
"errmsg" : "all members and seeds must be reachable to initiate set",
"ok" : 0
}

mongodb-linux-x86_64-1.6.1/bin/mongod --dbpath /root/data --fork --logpath log.log --shardsvr --replSet set2/ip-10-194-11-207.ec2.internal:27018
> use admin
switched to db admin
> rs.initiate()
{
"info2" : "no configuration explicitly specified – making one",
"info" : "Config now saved locally. Should come online in about a minute.",
"ok" : 1
}