Reuters and MSNBC report that a "failed explosive device" has prompted an evacuation of Times Square. Earlier tonight, a man fled from an SUV that was smoking and making popping sounds. Inside, FDNY found "explosives, gasoline, propane and burned wires."
Update: Mayor Bloomberg and NYPD commissioner Ray Kelly, who left the White House Correspondents' Dinner early to return to NYC, will address reporters at around 2am.
Update x2: A bow-tied Bloomberg said at a news conference that it was a t-shirt vendor who first notified the cops, the bomb squad discovered "propane cylinders, fireworks and timing devices" inside the Nissan Pathfinder, and an investigation is underway. Meanwhile, most of Times Square is still shut down. (You can see for yourself on this live webcam.)