I posted about NiXPS back in June when they release version 1.0 of NiXPS. They now have a beta version of NiXPS 1.5 available that includes support for viewing XPS files on Mac OS X. According to the NiXPS blog, they've developed their own native XPS renderer.
This is far from the first XPS implementation on a platform other than Windows — there's already XPS support across a wide spectrum of platforms — but it's the first viewer that I'm aware of (although I'm guessing it won't be the last!).