I have been using this script to check for updates on my Redhat systems for quite some time. Put this into your cron.daily, and you have a daily nag to update your system.

#!/bin/bash                                                                                       

#########
## Yum Check Update Script
##
## This script checks for system updates and sends email
## to sysmin team if there are any updates.
##
## Changelog
## ---------
## 24 Oct 2008 (Junhao)
## - Initial commit
##
#########

_CAT="/bin/cat"
_DATE="/bin/date"
_HOSTNAME="/bin/hostname"
_MAILX="/bin/mailx"
_RM="/bin/rm"
_TOUCH="/bin/touch"
_YUM="/usr/bin/yum"

HOSTNAME=`${_HOSTNAME}`
DATESTAMP=`${_DATE} +%Y%b%d-%H:%M:%S`
EMAIL=root
MAILSUB="RHEL Update Available for ${HOSTNAME} on ${DATESTAMP}"

TEMPLOG=/tmp/yum-check-update.tmp

${_TOUCH} ${TEMPLOG}
${_YUM} check-update 1> ${TEMPLOG} 2>&1

if [[ $? != 0 ]]; then
        ${_CAT} ${TEMPLOG} | ${_MAILX} -s "${MAILSUB}" ${EMAIL}
fi

${_RM} ${TEMPLOG}