[mkgmap-dev] Satnav for non-proprietary maps?

The other day I got to thinking-is there no such thing as a GPS/satnav that can display maps created in an open-source format, such as .pbf? In other words, a format that doesn't have to be reverse engineered? Perhaps an analogy would be an unlocked cellphone-you pay extra for it (or hack it), in exchange for additional flexibility. I wonder if there would be a market for such a thing. Mark