Another theory which is more disturbing is that a passenger / hijacker gained access to the Avionic Bay / E&E bay where they would be able to disable all communications. The E&E is accessible from the passenger cabin in the forward galley, under the carpet but requires a special screw driver to access it. Once inside the hijacker would have access to all the avionics. Here is a panoramic view of the E&E. http://hawkeyemedia.com/panos/777_Avionics.asp
This still would not completely explain what appears to be the programmed flight path which would indicate the pilot / copilot was coerced or involved in the hijacking. Also seems the Malaysian government knew this flight was off course and did not request assistance from other countries to track it until it was totally off the radar.
Lots of motive theories going on that range from corporate espionage, terrorism, insurance payout to cargo theft. There are very little real facts being released at this time which is leading to more and more speculation. Seems the authorities believe transponders were "deliberately disabled" before last communication where person in plane said "goodnight" and turned off all communications at that point.