Version 1 is online [via Andy]
The XML Paper Specification describes XPS documents. An XPS Document is a paginated representation of electronic paper that is described by an XML-based language. XPS Documents are open, cross-platform documents that allow customers to effortlessly generate, share, print, and archive paginated documents. Microsoft has published the XML Paper Specification and made it available royalty-free.
XML Paper Specification v.1.0 (October 2006)
XPS License Overview