"I think this was a test run. Now they know what they are capable of it could happen again." except they wouldn't get away with it again because the next time a plane disappears off radar they will be on to it quicker than you can say "missing plane,"
This thread is heading into the realms of hysteria now. There has been no indication that this has anything to do with terrorism or dirty bombs, and there is no way this plane could now be used to bomb a city because the instant it enters the sky it will be discovered by the military...
But actually, it could just create this kind of hysteria - make a plane disappear, then let the world speculate, and become terrified, over what might have happened to it. sort of mock terrorism...
personally the only plausible theory IMO is that it was highjacked for political purposes but that the highjacking went wrong and it ditched in the sea.
They wouldn't have twenty plus countries involved in the search if they "knew" something more than this. They wouldn't have taken a week to reach these conclusions if they knew there was terrorism involved. My bet is that the malasians are bumbling idiots in this regard and just don't really know what they're doing, and the fact the plane diverted quite so far away means that they can legitimately pass the buck to someone else who has more of a clue...