Terrorists are patient, think about the time span between the first bombing of the world trade center and 911.
Hehe, here is alan1's conspiracy number one.
All communication from the plane ended, transponders, emergency locator beacon, radio. While it is slightly possible that that could happen by accident, it is highly unlikely for all systems (and backup systems) to fail simultaneously. That would indicate that there was intent. If it was intentional, that would indicate hijacking.
Oh, and like I said, terrorists are patient. If the intent was to hijack a plane for future terrorist activity, then the terrorists would remain quiet about it.
So, continuing with the conspiracy theory, it was a hijacking for future use of the plane. Perhaps that future use is to put a dirty nuke in it then fly to and crash it into a large metro area. This is certainly not beyond the realm of a terrorist action. Again, terrorists are patient, it could be 3 days or a year before they use the plane.
Then we have the passengers. Terrorists wouldn't be concerned about the passengers. If they took control of the cockpit, they could easily depressurize the planes cabin. Those oxygen masks that drop would only supply oxygen for an hour at best. Stay in the air longer than that, and all the passengers are either dead or passed out by the time they land at their destination. They don't have to deal with fighting passengers after landing, just deal with removing the dead bodies.
Cell phones of the passengers were still ringing after the plane vanished. Some cell phone service will still present a "false" ring for a phone not on the grid, but many would go straight to voice-mail if the phone isn't connected. Too many cell phones rang, indicating that said phones were not damaged in an explosion or at the bottom of the ocean. It would take a while for terrorists to shut off all the cell phones.
Over 8 countries, over 40 ships, over 30 planes couldn't put Humpty Dumpty back together again, or find wreckage debris. Yes, I know the search area is large, but how often has five days passed with no sign of debris or wreckage from a plane crash? Rare indeed.
I posit that the plane was successfully hijacked, landed somewhere, the passengers killed and that it will reappear at some point in the future as a terrorist weapon. That's my conspiracy.