Addressing problems with Software Update in OS X
Software Update may sometimes not work properly, and either seem to miss specific updates or claim they are already installed and the system is up to date. People who have been affected by this have tried numerous approaches, including booting to Safe Mode, running permissions fixes, and even resetting hardware settings; however, nothing helps.
http://reviews.cnet.com/8301-13727_7-20004377-263.html?part=rss&tag=feed&subj=MacFixIt
