I think it's unlikely the plane was hijacked.
Here's what I think happened: Shortly after the last regular contact, the plane had some kind of drastic mechanical/structural failure which disabled the communications antennas (as well as other systems probably). The pilots immediately turned the airplane back towards their origin so that they wouldn't enter foreign airspace without communications and be shot down. This accounts for the rapid change of direction back toward Malaysia. Somehow in the confusion they got the plane into a steep climb and went to 45k feet (reported by radar) before losing consciousness. At that point all passengers and crew were incapacitated and the airplane put itself into stable flight, returned to cruising altitude and began powering down various mechanical systems (including environment) to keep itself flying (apparently this is what 777's are programmed to do under certain conditions). After that point it proceeded as a ghost-plane back across Malaysia and into the indian ocean where it ran out of fuel in the ocean somewhere south of Sri Lanka.
If any of the passengers had been conscious when the plane crossed land someone would have made a phone call. And we haven't heard anything about cell phone activity, so that pretty much guarantees that everyone on the plane was unconscious. I can't imagine any level of hijack takeover which could prevent over 200 desperate people from activating a cell phone, and I also can't see any reason for a well planned hijacking to take an airplane straight into the Indian Ocean just to let it run out of fuel. Makes no sense.
I'll probably be proven completely wrong tomorrow, but that's the most sense I can make out of the vague and conflicting evidence presented by public media so far.