Altova's XMLSpy now has rich support for working with the Open XML formats. As they say in their press release:
XMLSpy provides powerful support for accessing, editing, transforming, and querying XML data saved in Microsoft® Office 2007 documents and other zipped files.
Check out this link for more details: http://www.altova.com/features_office_2007.html
Navigate parts and relationships:
Directly edit the XML of any part, and get validation and auto-complete: