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.
A hijacking gone wrong is really the only reasonable guess at this point, at least based on the information that has been made public. I doubt the plane is still intact, much less carting around stolen nukes or anything crazy like that.
Authorities currently believe the southern arc, through the Indian ocean, is more likely than the northern one. Makes sense, based on what we know -- if the plane kept flying for 5+ hours and passed over land, you would think that it would have turned up on local radars. CNN also mentioned that the pilots are under increased scrutiny, with searches being executed on their homes (although I'd imagine that's just desperate scrambling for clues rather than evidence that they're guilty of wrong-doing).
