Microsoft released Service Pack 1 to update Office 2008 for Macintosh on Tuesday, May 13. The update makes a significant number of changes — spread across Entourage, Word, Excel, and PowerPoint — so if you are a system administrator, I would recommend examining the full description of Office 2008 for Mac Service Pack 1 (12.1.0) in its release notes.
The update is downloadable via Microsoft AutoUpdate (invoke that helper program with Help > Check for Updates in any of the Office applications, or look inside /Library/Application Support/Microsoft in the file system) as well as the Microsoft Office for Mac Web site.