In RH 6.2, syslogd used to log CROND crontab jobs to /var/log/cron
and the rest to /var/log/messages.
in RH7, cron is still valid but messages is flooded by things that don't 
have their place in it: CROND entries are duplicated, once in cron, once 
in messages, with the same config than in 6.2.

If I use something like :
[blabla];cron.none	/var/log/messages

and the crontab line stays :
cron.*			/var/log/cron

then CROND messages are no longer logged anywhere.
According to the documentation of syslogd and syslogd.conf, this should be 
considered as a bug.