It's a six hour flight to Beijing from KL and contact was reportedly lost at two hours in. That's not over water between Malaysia and Vietnam, so I discount the oil slicks. It's over northern Thailand or Laos, depending on the course set. If the plane went down, it will be on land and possibly in mountains. If it was taken over and forced to ditch, it will be elsewhere in the Gulf of Thailand or possibly in the South China Sea near the northern boundary of Vietnam.
If the plane came apart in the air or hit the water at great speed you would expect to find numerous small pieces of floating debris. No such crash evidence has been found, as I understand it.
AFTERTHOUGHT AND CORRECTION: I was relying too heavily on maps from the news services. After looking at a globe, I can see that a great circle route that was lightly modified to address sensitive issues of air space might have flown over water slightly to the east of Vietnam and then turned a little for a direct approach to Beijing. If the plane is in the water, it's in the South China Sea, not the mouth of the Gulf of Thailand where most news media have been placing it. I don't know where the suspect oil slicks are located. I would still expect to see floating debris -- clothing, seats, cushions -- if the plane had hit the water hard either as a unit or in pieces. We need some significantly improved reportage of this incident from people who actually understand geography, flight paths, and the number of miles or kilometers covered during a specific length of flight time.
FURTHER UPDATE SUNDAY MORNING. Now it appears that the 2-hour statement was in error. There are reports now that the flight disappeared from radar about 1:30 a.m. local time, or less than an hour after departure. This would put it over water in the area wher searches first began. Malaysian Airlines released a geographical position that is not widely reported in news accounts but made it into a China News Service and a BBC report that I could find: 065515 North (longitude) and 1033443 East (latitude). Now the terms latitude and longitude are reversed, which is alarming and may be the fault of a news organization rather than the airline, but the positional information by itself puts the disappearance over water between the Malaysian peninsula and the southern tip of Vietnam. So searchers are apparently looking in the right area.
There is some evidence from military radar that the plane may have turned back shortly before disappearing, and it is also reported this morning that a search operation was started off the west coast of Malaysia on the chance that the plane overshot the peninsula on an emergency return.
The known information may be getting a little better, but a lot of it is still confusing and hard to put together in a sensible package.
I continue to hold out hope for survivors, who may exist if the plane ditched and people could get out before it sank.