The other thing I'd caution you about when trying to monetize this is that the local bus companies will almost certainly come after you, and will have legal teams who are used to fighting the minutae of transport law - so don't be surprised if you find yourself having to lawyer up with some very specialized (and hence, expensive) counsel.
As an example of both, here's ([1]) a story of a Canadian startup that was declared illegal by the Ontario Highway Transportation Board (a tribunal no Ontarian outside of the transportation sector has ever heard of) after the bus companies when after them. Seeing as it's in Canada that case shouldn't serve as a precedent, but does (as does the UberCab story) serve as an example about the challenges that come up when you try and disrupt something as established as the transpiration sector.
[1] http://techcrunch.com/2008/11/12/ill-never-let-canada-live-t...
The taxi regulations only apply to taxis that can spontaneously pick you up on the street, not cars arranged for in advance. The gray area e.g. Uber is whether or not hitting a button 5 on your iPhone 5 minutes ahead of time counts as being "arranged in advance"