Bad stuff. I've been following the news most of the day on and off. Its almost certainly pro-Russian forces in Eastern Ukraine who mistook the airliner for a Ukrainian military cargo plane. Apparently a rebel leader posted on twitter not long after contact was lost with the plane bragging about downing another Ukrainian plane, and then promptly deleted it. I highly doubt they meant to shot down a civilian liner, its almost certainly accidental.
There were Dutch, Australian, Malaysian, UK, German, Canadian, Belgian, and Phillipine citizens on that plane. And there's still 47 or so passangers who's nationalities haven't been released yet. I know speculation is that 23 Americans may have also been on board.
This has the makings for a terrible international incident.