Vista use on the rise According to the latest statistics report on internet connected computers, collected by market research company Net Applications, the use of Windows Vista has risen to 14% of all connected computers. In comparison: in the same survey about 7,5% of all connected computers are Macs. You can see an evolution in the following graph (pulled from the Net Applications site):


This is significant for everyone who uses, and makes software for, the XPS file format.
Vista contains an XPS Viewer, and an XPS printer (MXDW) by default. So a 14% market share for Vista can directly be translated to '14% of worldwide desktops are XPS capable'. This expanding ecosystem is important, as it is one of the key characteristics of the XPS format.
An ecosystem is important for any electronic document. You can only reliably use it, if you know that your recipient can view it. Similar for generating it; you can only ask your recipient to send you a file in a given format, if you can safely assume that it is fairly easy for her to generate it.
By 'riding on the back' of Windows Vista, XPS is ensured of an every broadening ecosystem, making it a more viable, and capable, electronic document every day.  ¶ 08:53