see comment on http://www.mongodb.org/pages/viewpage.action?pageId=133409#comment-134885855 :

Looks like C++ driver for neither mongodb 1.6 nor 1.7 use pid to generate new OIDs under MacOS X, it creates problems for multi-process programs. As a simple workaround I suggest calling srandomdev() right after forking and before calling mongo::OID::newState() or mongo::OID::justForked()

before closing, please also investigate srandomdev() or alternatives on init of mongod and mongos so that getNonce is as good as possible.