Sep 30 02:06:34 guinness rhnsd[1355]: running program /usr/sbin/rhn_check Sep 30 02:06:37 guinness rhnsd[784]: command returned: ERROR: unable to read system id. every $INTERVAL minutes. This is _very_ annoying. It also makes newbies think, that they actually NEED such a system id (see guinness-list for an example).
A new version of rhnsd will soon be released which explicitly checks for /etc/sysconfig/rhn/systemid before running a subcommand. It will log at the DEBUG level what it finds, therefore not logging to syslog by default.