I often hail livery (aka radio) cars in NYC. It's technically illegal for them to pick up passengers that they weren't dispatched to pick up. I'm sure that is a law put in place to protect medallion owners.

As an aside, there are different schools of thought for how to pay radio cars since they don't have meters. A friend always waits until getting dropped off to negotiate price. He figures he holds the power at that point since he is already at his destination. I always ask how much before getting in. My girlfriend just hands them money when she gets out, whatever she thinks is fair, and has never had anyone give her a hard time.

EDIT: I should add that all three of us find the cost of the radio cars to be about the same as yellow cabs. Our different negotiating techniques don't really make a difference.