Jerry Dunietz is now up on Channel 9 talking about the XML Paper Specification. They use ZIP in a similar way to how we use it in Office to store our XML files. In fact, the APIs that they release for parsing through the contents of their packages (files) will also work against an Office XML file. Of course the contents within the ZIP packages are different, and the schemas used are different, but the concept of parts and relationships within the package are the same. I talked about this a bit the other month in this post: https://blogs.msdn.com/brian_jones/archive/2005/06/06/425750.aspx
Jerry and Tim also talk briefly about Office's use of these conventions as well as the fact that anyone can actually come along that is building an application and leverage the container conventions for their formats. It's all just ZIP and XML so it's pretty straightforward.