One main hurdle to get over is the cabbies figuring out how to accept bitcoins themselves (so they can keep certain transactions off the books)...
That's why the app should be probably "AirBnB-style" - why can't ANYONE be its own taxi in your BTC-based system? Feedback system takes care of reputation while you're just providing info/metering services and take a small cut for this. Highly scalable, global expansion only a matter of localization/geo services availability.
Start in major metro areas, spread like a wildfire. Disrupt the whole rigid industry with their "medallions", "taxi licenses" and all such nonsense.