The Microsoft Office 2008 12.3.3 update contains a critical security fix, as well as significant improvements in stability, according to Microsoft's release notes. Office 2008 apparently contained a flaw that could be exploited to gain control of a computer remotely. An attacker would have to create a specially crafted Word or Excel file that a user opened. No reports of this attack, labeled by Microsoft as privately reported, have been heard of in the wild.
The hefty update also improves stability of all the Office 2008 applications and many components. For those of us who work on Take Control ebooks, seeing the note that in Word 2008, stability is improved when "you accept tracked changes in a document" is music to our ears - we use change tracking constantly while editing documents and have seen regular crashes during revisions. (Free update, 349.5 MB)