Microsoft has released an updated beta of their Open XML File Format Converter which now supports .docx, .docm, .pptx, .ppsx, and .potx. Still no sign of an Excel (.xlsx) converter.
Download the latest version of Microsoft’s Converter for Mac.
Note: You will need MS Office 2004 or MS Office v.X.
Update:
Alternatively, you can download the free NeoOffice, a Java-based office productivity application which can read/write in the open XML format.
Recently found a great online converter, it actually works with docx and pptx files.