Whilst you don't have to use any Microsoft tools or technology to create, manipulate or consume OpenXML files, Microsoft does provide a library for accessing these files as part of the .Net 3.0 (aka WinFX) technologies in the System.IO.Packaging namespace...
Another TechED OpenXML related announcement Altova XMLSpy Now Supports New ECMA Office Open XML File Formats June 4, 2007; 05:51 AM Altova ( http://www.altova.com ), creator of XMLSpy and other leading XML, data management, UML, and Web services tools...