Based on note & i/m exchange with scotthernandez:

• The configdb option (order of the config servers) is not fixed forever, only for a running cluster
  • All mongos must use the same configdb option (same order)
  • The first (avail) config server listed is used for reads
• The configdb option (with the order) can be changed, but only if all mongos (and shards) are brought down and then up with a new configdb string.
• Only mongos and the shards keep track of this setting
  • The actual config servers know nothing about the order, or each other

http://docs.mongodb.org/manual/reference/mongos/?highlight=configdb#cmdoption-mongos--configdb