Slippery Oil Price

What in the world is going on with the price of oil? Just a few months ago, in June 2008, it was $140 a barrel and now it's less than $97. The high price was justified because of demand, but what justifies the drop? Surely, the demand has not dropped 40% in four months?

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Surely it has nothing to do with keeping Oil as the primary energy source? With a oil price above $130, it makes economical sense to develop alternative sources of energy. With the oil price dropping, this might not be the case anymore. So the investments in alternative energy sources will come to a halt. And oil remains the primary energy source.
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Zaaf