Nissan is providing the vehicles, along with some fast chargers, so the test fleet doesn't run out of juice and cause even more traffic gridlock (I live here, so I know about gridlock). According to Mayor Bloomberg's office, this EV taxi pilot "will enable Nissan, the city, the taxi industry and the public to learn more about how to integrate electric vehicles as taxis."
The Leaf, as we all know, is the world's best selling five passenger all-electric zero-emission vehicle. Nearly 60,000 Leafs have been sold globally, but not many in New York City, where few apartment buildings have garages and few public garages have charging stations.
The pilot Leafs will be made available to both individual owner-operators and taxi fleets, which must have space to install the chargers, be interested in new technology and be willing to provide the required feedback. There's a little more information in this PDF from the governor's office.
The Nissan Altima and Altima Hybrid already are popular as NYC taxis. The Nissan so-called "taxi of tomorrow" NV200 van is expected soon, too.