This happened to my parents on a US flight HPN-DCA in October. About 15 minutes before landing, they were told there was a "security problem" with the flight and they were being diverted to IAD. The plane parked far from any IAD terminal and was surrounded by police cars. Police boarded the plane and escorted the pilot off. They were given no information for about 30 minutes, before they were finally told by the embarrassed pilot, who had reboarded, that he didn't know the password. My parents were scared out of their minds. They were given the choice of deplaning at IAD or remaining on board to fly to DCA. According to my mom, US was not particularly nice about the whole thing, offering no help with ground transportation for people like my parents, who had no idea how far IAD was from DCA, what taxi service was like, etc. I'm not going to join in the debate about the ridiculousness of the whole password procedure, but the whole diversion thing could have been handled a whole lot better, IMHO.