Russian President Boris Yeltsin returned to the United States to prepare for his fifth meeting with President Clinton -- this time a formal Washington summit with arms reduction troubles on the agenda. This afternoon, Yeltsin took center stage at the United Nations in New York four hours after Clinton did, telling assembled diplomats, "Today, Russia is a threat to no one." He also signaled an apparent turnabout from last week's prickly exchange after U.S. officials accused Russia of dragging its feet on the last START arms treaty. Now, he says, a "serious and fruitful dialogue" on permanently dismantling warheads is in store. But not before Tuesday night, when Yeltsin, Clinton and hangers-on don white tie and tails for a White House bash. When the boys get back to work, they're expected to sign an agreement on dismantling nuclear weapons.