ECMA International has started a technical committee, TC46 - XML Paper Specification (XPS), to “produce a formal standard for an XML-based electronic paper format”. One of the points in the TC46 programme is to consider a submission of the XPS format as an ISO standard.
This is really good news to all users of the XPS format and is really vital if XPS is going to be able to compete with the PDF format (which already is an ISO standard, ISO 19005-1:2005/ISO/DIS 32000).
TC45 - Office Open XML Formats, the XML based format used in Office 2007 is currently in the ISO approval process and if all the ISO members approve of it, as ISO/IEC 29500, then XPS will probably have a good chance in the possible ISO process. Office Open XML and XPS share a lot of common grounds, such as the way the documents are packaged.
You can read more about the XPS format in my Dissecting XPS series.