Microsoft has released a "critical" Office 2008 update, 12.1.1, which "contains several improvements to enhance stability and performance."
Several issues are fixed with the updates, including charting problems, Entourage crashing when the computer wakes from sleep, and issues regarding converting documents to and from the Open XML Format. A complete list of improvements is available in the Microsoft Knowledge Base.
The update can be installed via Microsoft AutoUpdate (accessible by choosing "Check for Updates" in any Office application's Help menu) or from Microsoft directly. The update is a 153MB DMG file, and is available in 11 languages.