I havent tried this yet, but my messages file gets filled with these messages too. This kind of logging fron cron/pam is abnormally if you ask me.
Try deleting the "authpriv.none" bit of the /var/log/messages line in /etc/syslog.conf and see if that goes away.
That doesn't work either unfortunately.
According to my interpretation of the syslog.conf syntax it means that your suggestion will log all the authpriv messages to /var/log/messages.??
I hope someone else can shine a light of the logging issue of pam, these messages are too many.
Edwin