Since this seems to come up a lot, here is a pic of the elusive Borgnine Taxi.
You get it by completing 100 fares in the "Taxi Driver" side mission. The fares can be gotten at any time in any area of Liberty City, and do not need to be consecutive. Once you get enough fares, the pager will let you know that you can pick up a new model in Harwood. The Borgnine Taxi Company is located just to the north of the Diablo's turf (the projects), by the HEAD Radio station.
It basically has the speed and handling of the smaller Taxi combined with the armor of the Cabbie. The spikes on the bumpers also take out pedestrians on a single pass, even at low speeds.
Some trivia: the cab is named after Ernest Borgnine, who played "Cabbie" in the movie Escape from New York. And, sadly, you can't throw molotovs out of the roof like Cabbie does in the movie -- though that would certainly make a great mod.