mongos also returns true:
anden(mongos-2.5.0-pre-) test> db.isMaster()
{
  "ismaster": true,
  "msg": "isdbgrid",
  "maxBsonObjectSize": 16777216,
  "maxMessageSizeBytes": 48000000,
  "localTime": ISODate("2013-04-11T12:50:32.367-0700"),
  "ok": 1
}