Open XML Converter for Office 2004 and 12.1.1 Update for Office 2008

Stuart: Thanks for pointing that out! For everyone else, here's the link the the Microsoft Office 2004 for Mac 11.5.0 Update

Microsoft also released Office 2004 11.5.0 update, which according to Microsoft:

Microsoft Office 2004 for Mac 11.5.0 Update

This update contains several improvements to enhance stability and compatibility with documents that were created in Office 2008 for Mac and Office 2007 for Windows. If the Open XML File Format Converter for Mac is installed, this update enables Office 2004 for Mac to read and write Office documents that are in the Open XML Format. For more information about this update, please visit the Microsoft Web site.

Applies to: Office 2004 Standard Edition, Office 2004 Student and Teacher Edition, Office 2004 Professional Edition, Word 2004, Excel 2004, PowerPoint 2004, Entourage 2004.
Released: Tuesday, June 24, 2008