Thinking aloud about this.
I have used a Microtek USB GPSR module with laptop computers running OpenSuSE GNU and Darwin. Navit, an open-source GPS-based navigation system, offers routing, speech output, and a number of other nice features. It would not build on Darwin, however, in part, because of endian issues. It would be nice if it would build on whatever flavor(s) of GNU will be running on the Raspi, using whatever endian-ness the Raspi uses.
User interface would be tablet-like, using a touch-screen, similar to commercial units such as Garmin Nüvi or Tomtom.
What version of Windows[sup]Ⓖ[/sup] do I run? Why, X11 R6, of course!