SharePoint office server and WSS 3.0 December cumulative update is now available for download. Check detailed installation instruction from SharePoint Team Blog first:

http://blogs.msdn.com/sharepoint/archive/2008/12/17/announcing-december-cumulative-update-for-office-sharepoint-server-2007-and-windows-sharepoint-services-3-0.aspx

December Cumulative Update for Windows SharePoint Services 3.0

December Cumulative Update for Microsoft Office Servers