In a small but significant philanthropic gesture, Warren E. Buffett is opening his huge wallet to help support a group founded by Ted Turner and former Senator Sam Nunn whose aim is to reduce the threat of nuclear, biological and chemical weapons. Calling the threat posed by nuclear and other unconventional weapons "the ultimate problem" confronting mankind, Mr. Buffett said yesterday that he had decided to give the group, the Washington-based Nuclear Threat Initiative, $2.5 million over five years and become an adviser to its board. His commitment to the group is to be announced on Friday. "It's not that...