With Hurricane Irene pointed at the nation's most populated city, expected to arrive by late Saturday, New York Mayor Michael Bloomberg wants coastal residents to get moving. Now. City officials made the unprecedented decision to shut down the mass transit system starting at noon Saturday and to evacuate by 5 p.m. more than 250,000 New Yorkers who live along the southern coastal rim of the city. That includes residents of Manhattan's Battery Park City, vast parts of Staten Island, Brooklyn's Howard and Manhattan beaches and everyone in the beachfront communities of Rockaway.