Not much to say about this one. With Microsoft Office 2008 Service Pack 2, a bug was introduced that prevented all the Office applications from opening some of Microsoft's Open XML documents, templates, and macro-enabled documents and templates (see "Microsoft Releases Office 2008 Service Pack 2 [1]," 2009-07-20). Several commenters [2] on our article about Service Pack 2 had the problem, though all the .docx and .xlsx documents on my Mac seemed to open fine.
Microsoft has now released the Microsoft Office 2008 for Mac 12.2.1 Update [3] to fix this problem. In my initial testing, it was still able to open the Open XML documents - if you weren't seeing any problems before, it's probably not essential that you upgrade immediately. The update is a 23.8 MB download, and is available from Microsoft's Web site or by choosing Check for Updates from the Help menu of any Office 2008 application.
[1]: http://db.tidbits.com/article/10375
[2]: http://db.tidbits.com/article/10375#comments_311
[3]: http://support.microsoft.com/kb/974170