OK, you guys ready for a real (lengthy and detailed) theory?
First, there is little evidence that supports the idea that this plane is at the bottom of an ocean. The idea is somewhat plausible; however, for it to be true the aircraft would have needed to turn 90 to 120 degrees to the South after overflying two navigation fixes in the Strait of Malacca which clearly showed it on a course to the NW not SW and that it was probably being navigated by someone who knew what he was doing. Assuming the aircraft did turn South and fly for five more hours then it is indeed on the bottom of the ocean since there are only two islands (Christmas and Cocos) in that part of the Southern Indian Ocean and it isn't at either (and it certainly couldn’t reach Antarctica). So, why would the aircraft turn South given that there was no place to go? If the intent was to simply crash it then why bother with any of this? If the plane was essentially "unmanned" then how did it make two major heading changes (the first turn to SW after the transponder was turned off and the second turn to the NW in the Straight of Malacca) seen on the Malaysian radars and, if not controlled by that point, who would have turned the aircraft to the South to send it into the Southern Indian Ocean? No, there's no evidence supporting a course to the SW which leaves us with NW, the final observed course as the plane left the radar coverage in the Strait of Malacca.
The Indian Ocean INMARSAT satellite received the aircraft's "pings" and is capable of measuring the signal’s elevation angle relative to the satellite but is not capable of giving a line of bearing. The final “pings” at 8:11am were on the 40 degree elevation arc from the INMARSAT satellite which means that the aircraft had to be somewhere on that arc when it went down (or landed). That arc stretches from Northern Laos through Tibet and the Gobi Desert to its furthest end in Kyrgystan which is also the absolute maximum range of the aircraft given the fuel on board. All of this arc is overland. Look along that arc and you'll also see that it's all mountainous or forested except for parts of Tibet and the Gobi.
The final "ping" was at 8:11am Malaysia time and five-and-a-half hours after the plane was last seen on radar headed NW over Pulau Perak island in the Strait. How far would the airliner have flown between 2:40am and 8:11am? Well, airliners typically cruise at about .85 Mach which equals (at 35,000 ft) 7.6 miles a minute in no wind conditions so we're looking at a maximum range of about 2,500 miles in that 5.5 hours. The straight line distance from Pulau Perak to where the 40 degree satellite arc crosses the Gobi is about 2,300NM so, theoretically, the plane would have overflown the desert but that’s in a “no wind” condition and assumes it flew directly from Pulau Perak to its landing spot. The reality is that it would have been flying into a headwind for at least part of the flight which would have reduced its groundspeed and therefore the distance flown in 5.5 hours. Assuming the plane continued NW for a while to ensure it stayed clear of the ground radars in Thailand and the Malay Peninsula before turning more Northerly the distance flown would also be longer than a direct route and, once arriving, it would still have to slow down, locate the landing site, probably reconnoiter the site, and then set up for a landing all of which would have taken some time. All of this is perfectly feasible. It’s certainly possible that the plane made it as far as Kyrgystan but given that Kyrgystan is mountainous it would be more reasonable to land it on a dry lakebed in the Gobi especially given that the Gobi is the largest desert in Asia (measuring 1,000 by 500 miles) with lots of places to land a plane while being very sparsely inhabited by nomads and free from prying eyes. Also, let’s not forget that while the aircraft took off in the middle of the night the landing would have been right after sunrise making a visual landing on something like a dry lakebed a simple proposition.
So, why no ransom call yet? Well, it’s possible that the airplane simply ran out of fuel and didn’t make it to the intended landing area (even though it was over land) or that it crashed on landing making any ransom demand irrelevant. But, it’s even more probable that the hijackers would need time to camouflage the plane, clean up the landing area (get rid of tire tracks for instance) and, since they wouldn’t want to be located electronically, they wouldn’t have simply phoned in a ransom demand from the site. Someone would have to move well away from the plane say, to South America, and they’d have to be able to furnish some sort of proof of life. Also, although nothing has been said publicly, would it surprise anyone if the investigators have received all sorts of bizarre claims that they wouldn’t make public? Maybe they’ve already been contacted but haven’t had reason to believe that particular claim. The hijackers would have to provide “proof of life” to be taken seriously which means they’d have to physically transport some evidence from the landing site through a courier to another location and set up a safe means of transmitting the demand and proof to the target. If you stop and think about it, given that they went to great lengths to avoid tracking of the aircraft (but obviously didn’t not know about the “pings”) they probably assumed that there would be almost no chance that the landing site could be located so what’s the rush? Actually a delay would help increase doubt as to the plane's possible location as the longer they waited the more likely it is that the plane could have been refueled and moved. From a practical standpoint, the passengers would be in the middle of the largest deserts in the world so where would they escape to? All the hijackers would need to do is keep a small cadre on-site to control the passengers and give them minimal rations to sustain them.
That’s my story and I’m sticking to it. Anyone want to make any bets?