Dang, and at cruising altitude it does not bode well for survivors, not that miracles don't happen, but the odds are against it.
It'll be the first 777 to be lost whilst at altitude, the other hull-loss incidents have all been on landing or on the ground. If there was no indication of trouble before loss of contact then it must have been a massive structural failure...deliberate or otherwise.
Unfortunately Flightradar24s coverage has the aircraft contact lost half-way between Malaysia and Vietnam, in the sea:
There should be a press conference shortly.