Currently uses int64:

m31000| Mon Aug 22 12:46:09 [conn4] command admin.$cmd command:

{ getLastError: 1.0, w: 7.0, wtimeout: 30000.0 }

ntoreturn:1 reslen:94 6143ms
{
"n" : 0,
"lastOp" : NumberLong("5643722588996763649"),
"connectionId" : 4,
"wtime" : 6143,
"err" : null,
"ok" : 1
}