Balancer seems to have gotten stuck after acquiring a lock which then stopped all balancing until we logged into the mongos and stop/started the balancer.

mongos log-
Fri Jan 18 00:45:02 [Balancer] distributed lock 'balancer/ms-db-4:27017:1354112926:1804289383' acquired, ts : 50f8fd7e8d5ca2b8deff6383

use config;
mongos> db.locks.find(

).pretty()
{
"_id" : "balancer",
"process" : "ms-db-4:27017:1354112926:1804289383",
"state" : 0,
"ts" : ObjectId("50f8fd7e8d5ca2b8deff6383"),
"when" : ISODate("2013-01-18T07:45:02.249Z"),
"who" : "ms-db-4:27017:1354112926:1804289383:Balancer:846930886",
"why" : "doing balance round"
}

we didn't notice this until today that we had no balancing happening.