Koen Zwikstra has posted the source code for his version of a Silverlight XPS viewer for Silverlight 2 and now you may download it and use it for whatever you want.
Some thoughts from Koen: “First of all, since the viewer employs TextBlock instead of Glyphs elements, it is likely that the position and content of generated text elements are incorrect. This basically renders the client-only XPS viewer useless for any production environment. I'm working on an actual solution.”