If you’re using yum-cron and want to automatically reboot your CentOS box whenever the kernel gets updated you can add this code to /etc/cron.daily/0yum.cron right before exit 0 at the end:

entry=`cat /boot/grub/grub.conf | grep '^default' | cut -d '=' -f2`
entry=`expr $entry + 1`
if [ "`cat /boot/grub/grub.conf | grep '^title' | tail -n +$entry | head -1 | sed -e 's/.*(\(.*\)).*/\1/'`" != "`uname -r`" ]; then
sleep 10 ; reboot
fi

This was taken from here with a tiny correction displayed in bold above.

Depending on your version of CentOS you may want to adjust /etc/anacrontab or /etc/crontab to set the times when cron.daily and therefore yum-cron will be run to avoid reboots due to a kernel updates in the middle of the day.