Everyone seems now in agreement (with the possible exception of some Malaysian officials) that Flight 370 continued flying in a generally westward direction for four or five hours, at a minimum, after it was lost by civilian radar shortly after takeoff. Substantially all observers also agree that measures were taken to hide the plane’s whereabouts, e.g. by disabling the transponder. So it seems clear that the plane was hijacked, either by terrorists or by the pilots themselves (or perhaps one of the pilots was in league with terrorists among the passengers). The fact that whoever was in control of the airplane knew how to disable the transponder and to keep the plane in the air for at least four hours suggests that he is or was an able pilot and could have intended to land the airplane (unlike, for example, the 9/11 hijackers).
So, while we have absolutely no idea where the Malaysian aircraft now is, we have a pretty good idea what happened to it, up to a point. There seem to be three main possibilities:
1) The plane may have crashed into the sea, due to pilot error or running out of gas (the Payne Stewart theory?). Initially there were thoughts that this could be an instance of pilot suicide along the lines of the Egypt Air crash, but it seems unlikely that a pilot who intended to crash his airplane into the sea would fly it for four hours first. If the plane did crash into the Indian Ocean, most likely accidentally–or possibly as a result of a Flight-93 type passenger revolt?–debris will eventually be found.
2) The hijackers may have prepared a place, perhaps on an island in the Indian Ocean, possibly in eastern Pakistan, to land the airplane, and they may have landed it successfully. This would not be easy: a Boeing 777 needs a runway around two miles long. But it may be possible. If that is the case, then why haven’t we heard from the hijackers? If they are terrorists (e.g., radical Muslims) they may intend to use the airplane in a terrorist attack and will not reveal themselves until they have an opportunity to do so. It has been speculated that the hijackers might have been criminals rather than political terrorists, and their objective may have been to land the airplane safely, and then ransom the plane and the more than 200 passengers. On this scenario, however, they would want to get the ransom underway as soon as possible, and we presumably would have heard from them by now.
3) The hijackers may have prepared a place to land the airplane, but failed to land it. If the airplane crashed while attempting to land, the debris will eventually be found, and its location along with other evidence (i.e., a nearby landing strip) should make it apparent that this is what was going on.
It appears that something truly extraordinary happened to Flight 370. My own guess is that within the next week or two, wreckage from the crash will be found somewhere in the Indian Ocean. But it is possible that we may never fully understand what happened on board Flight 370, and why.