Since we're all blindly speculating, I may as well throw in my best guess. I'd say the most likely scenario is that it went tits up for the hijackers. Some time after they cut communications and left radar range, something or other went wrong (passenger riot, a la that would-be 9/11 attack that crashed in Pennsylvania?) and the plane went down before it could reach its destination. Hence why nothing whatsoever has happened, no demands, no 9/11 repeat and nobody taking credit.