Well, that's a great question and I can only thank myself for asking it.
The Peer to Peer nature of Navizon is more an analogy than anything else. Take the file sharing networks like Kazaa, eDonkey or Bittorrent. It only takes one person to buy a CD and make it available on the network for everyone else to be able to listen to it.
With Navizon it's pretty similar only that it's with GPS: it only takes one person to map a city with a GPS for everyone else to benefit from a free software-only positioning system without having to buy a GPS. And it's 100% legal.
So is it really P2P? Well not exactly... If you're a purist you might want to call Navizon a peer produced and collaborative positioning system. But we just thought P2P was easier...
Navizon runs on Windows Mobile devices and can be downloaded for free here.