One of the features of XML Paper Specification, XPS, is the light-weight reader approach and the portable format which Adobe have had the major market share for, and still has, with the PDF format. We all like the approach with having some kind of document format that can be sent to anyone without worrying that they don't have a reader for the document.
I like that there now are competition on this market, even though I really think that Microsoft with XPS have a long road ahead until they reach an acceptable level of XPS users, even with an XPS reader built in to Windows Vista.
PDF is today the de facto standard for digital documents and since january this year Adobe has been working with an ISO technical comitee to submit PDF as an open formal standard (ISO 32000).
I had no luck searching the ISO site for any reference to the ISO 32000, but I easily found the ISO/IEC DIS 29500 which is the proposed standard for Office Open Xml which contains the basic functionality for creating XPS documents.
This topic is currently pretty hot and for your information here is a good article on the subject XPS vs PDF worth reading: With XPS as PDF killer, Microsoft opens second front on Adobe from Computerworld. The article discusses the main topics in where XPS has an advantage over XPS, such as making XPS more printer friendly by avoid the usage of for example PostScript.