A while back, I wrote of my conversation with a friend who was convinced that oil prices are set by a small cabal of traders, and that while they have been at $60-70 over the last several years, they would have been at $40 or less without the traders manipulating the price. I won't go into my arguments again here, but I wrote that it would be virtually impossible to maintain a price artificially above the market clearing price for so long without 1) massive product gluts or 2) almost-impossible-to-hide widespread suppression of production involving thousands of parties.
Michael Giberson at the Knowledge Problem writes about a price-fixing case by natural gas traders. Amaranth is accused of manipulating gas prices, and I won't judge their guilt or innocence. But, apropos of my statement above, it is interesting to note that the key question is whether it was possible for the company to manipulate commodity prices for eight minutes. It looks as if they tried it, but it also looks as if they were not successful (since the government is charging they attempted to manipulate the market, but is not trying to prove they succeeded in doing so). Making it hugely absurd to think that anyone could do it for three or four years.