I was thinking about a similar idea and also the parallel concept of "AirBnB for passenger seats", rather than unused towncar inventory, an easier way to "book" a carpool ride from someone who's already going that way. You just run into a ton of tangles around insurance, carpooling vs taxi licenses, scaling especially abroad, and the fact that one bad driver - one accident - could completely ruin your entire business.
I do think there's potential in disrupting the taxi business, though my current angle of thought is more about using technology, especially cleantech and mobile bookings, to make it a low cost planet-friendly business. Just the thought of all those exhaust fumes scares me. Go EcoTaxis!