At the moment I think that it was hijacked by people who were experienced pilots, either passengers or crew members (I don't suspect the Co-pilot, he was planing to get married) or a combination of the two. Currently, I think that some passages might have been involved.
I think that the governments know more about where the plane is than they're telling the media. I think that the plane and all of it's passengers are probably being held hostages, and that it has been landed, since their phones were ringing. The government can track mobile phones, so I think that they have found the plane but don't want to cause panic. This could all be wrong, at the moment I'm just speculating (this is what i do when I want to find something out but the information isn't there). There hasn't even been a reported hostage demand, then there also haven't been any distress signals. It might take years to know for certain what happened.