Adm. Mike Mullen, chairman of the US Joint Chiefs of Staff, says he works for only one president at a time, and that is true. But when the man who will be your boss in a couple of months asks for a meeting, it is smart to show up. Mullen, the senior US military officer, flew to Chicago, Illinois, for an unannounced face-to-face with President-elect Barack Obama to talk about security. A military official said Friday's sit-down in Obama's office lasted 45 minutes. It was a "get acquainted" session, said the official, who would describe the private meeting only on condition of anonymity. The official would give no other details, so it is not clear whether it was more strategy session or job interview. Mullen told The Associated Press last week that he expected to keep his job once Obama takes over.