A default RHEL4 install of a Rackspace (*) server contains a cron.d file named /etc/cron.d/rs_rhncheck that runs a job to check for Red Hat Network updates. In the default configuration this would send out a message every day indicating that up2date did nothing. To only get email when there is something interesting happening I changed the cron.d file to call the following script. The checksum is compared against the checksum of the output of up2date when it does nothing (I guess I could have tested the return code for diff). So when up2date either installs a package or decides not to install a new kernel package (or other important package) then I get an email.
/usr/sbin/up2date --nox -u > ~/up2date.out
SUM=`md5sum ~/up2date.out | cut -f1 -d\ `
if [ $SUM != "65c57b05b24bd8f656a0aec0d6db725a" ] ; then
(*) Rackspace support is really good. If I was paying I might look at other options but for my clients I am happy to recommend them without hesitation.