The Open XML Formats used by the 2007 Microsoft Office System share a common foundation with XPS: all the formats are based on Open Packaging Conventions (OPC). OPC defines a structured way to store the content within a defined container, with relationships that enable easy identification of and access to relevant parts within the container. In addition to providing complete support for XPS, the .Net Framework includes support for OPC within the System.IO.Packaging namespace. However, until now, there was only explicit support for the XPS format for .Net developers, other formats built on OPC, like the Open XML Formats, required the developer do additional work to process the file format.
The Office Platform team have released a technology preview of the Microsoft SDK for Open XML Formats. Chris Bryant has more details in a channel9 video, or you can go grab a copy of the SDK from the Download Center and play.
For more details on Open Packaging Conventions see part 2 of the Ecma Office Open XML standard (ECMA-376, December 2006).