"The XMLPaper Specification (XPS) was originally created by Microsoft and then adopted by ECMA TC46 as ECMA-388, the Open XML Paper Specification. This is also referred to as OpenXPS. XPS has the file extension of XPS and OpenXPS uses the OXPS file extension. These two file formats aren’t the same. Windows Vista and Windows 7 both contain support to view and create XPS files. You can choose to print to the “Microsoft XPS Document Writer” (MXDW) to create an XPS file and use the XPS Viewer to view them.
Windows 8 can view and create both XPS and OXPS files. See OpenXPS Support in Windows. The “Microsoft XPS Document Writer” defaults to creating an OXPS file [GD:Emphasis added]. When creating a document, you can choose which file type to create when you are prompted for the filename. You can change this default using the group policy editor or use PowerShell as described in Changing the Default Format for Microsoft XPS Document Writer.
You can’t view an OXPS file on earlier versions of Windows (i.e Windows 7, Vista, or WinXP) because, these operating systems only support XPS.
The Microsoft XML Paper Specification Essentials Pack (version 1.2) provides support for Vista and Windows XP to view and create XPS files but, not OXPS files.
However, you can use the XpsConverter utility to convert OXPS files to the XPS file format. TheWindows Driver Kit 8 (WDK) which used to be called the DDK, describes the XpsConverter tool. This tool isn’t included in WDK 7.
I hadn't seen this and personally had thought XPS was pretty much dead. Interesting that not only is it not dead on Windows 8, but there's a new, better, and more open version.
The important point here is that if you're on Win8 and using XPS to share documents (say you're printing them to XPS to share them with someone else, or to archive a print out, etc) and they are on Win7, Vista, XP, you're going to want to either change your XPS Printer Driver settings or convert them.