I know a bit (not a great deal, mind) about planes and stuff.
The Boeing 777 (be it a 200 series or 300 series, or any derivations thereof) is a pretty pukka bit of kit. It definitely took off and was tracked up until a certain point, when it went invisible. FlightRadar24 can probably replay some of the journey (I haven't checked).
What happened must have been catastropic - therories about a gradual loss of cabin pressure are unrealistic as there are so many sensors that set off BIG alarms that they would would have been impossible to ignore. I think some planes in these curcumstances will automatically reduce height to about 15000ft and switch their Squawk to 7700 (emergency) as well. This would have been picked up easily. And a loss of cabin pressure would not have turned the tracking device off - this can only be done manually or if it gets broken. If not turned off manually then it must have been broken by something; be it a bomb, complete disintegration due to a massive and sudden structural failure.
Talk that it may have flown on for hours are unlikely as it would only be carrying enough fuel for its scheduled stop plus 2 emergency sectors thereafter. Fuel is expensive - 1/3 of the cost of the flight - it wouldn't have been carrying any more than it needed to as more take-off weight=bigger fuel burn=more money=inefficiency.
So....I think it must have completely broken to smithereens at 35,000ft. It would explain why no bits have been found as they would have scattered far and wide and sunk within hours and/or too dispersed to make finding them realistic.
Bomb or bad construction? Boeing had better hope for the former. In fact I'm sure Boeing will have a big say in making sure it's the former, evidenced or not.
