In many cities there's a distinction between medallion taxis—which you can hail off the street—and livery service—which you need to call for. Since you generally have to take whatever you get get when you hail a cab, the prices are regulated and, in return, the competition is limited.

Generally, the number of livery service drivers and cars is not limited.

That said, the Washington, DC taxi commission has been hassling the Internet-based Uber. Last month they ran a sting against an Uber-arranged driver and impounded his car.[1]

[1] http://www.washingtonpost.com/blogs/mike-debonis/post/uber-c...