Deffo a high-speed near-vertical impact (very compact wreckage area and total airframe destruction). A very credible witness on BBC news described what looked like a 'roll / loop' into the ground, which fits the pictures of the accident site.
So, unlikely to be carb heat or a wire-strike in a precautionary landing (though it did slice the wires on the railway as it went in).
I have a couple of likely theories, but that's all they are, so I'll keep them to myself for now. But a bit of imagination and a bit of looking at past accidents will show what these might be.
What a terrible start to '09.
SSD