https://github.com/mongodb/mongo-ruby-driver/blob/1.8.3.rc0/lib/mongo/util/sharding_pool_manager.rb#L28

Using @client['config'] fails if authentication is required at the mongos.
more, it fails silently due to the

rescue Mongo::OperationFailure

block below...

We did a patch and its now working

config_db = @client['config']
config_db.authenticate('...','...')
seeds = config_db['mongos'].find.to_a.map{|doc| doc['_id']}

Thanks
Boaz